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For MPICH derivates and Intel MPI, we can improve the compiler wrapper
behavior and accuracy by defining certain environment variables if they
haven't been set otherwise.

If our ``find_library`` step hasn't used a particular link directory at
all, it's best to retain it in order to prevent issues from secondary
dependencies not being found.

This replaces hardcoded expectations of flags like -l and -L with a
dynamical solution based on CMake platform variables.

Furthermore, the linker flag parsing is dynamified to support more forms
and given linker paths are now removed properly from the linker flags.

The parsing of link information coming from the compiler wrapper has been improved:

- Support MSVC /link argument separation properly and add support for potential VC++ link flags
- Rely on the global import/static/shared library suffixes instead of hardcoded special values.
This should improve compatibility with Cygwin and MinGW should any MPI implementation there need this behavior.
- Don't use ``find_library`` if the full path of a library is known anyways.

Move the logic to search mpiexec for MSMPI and MPICH2 guesses to their
respective guessing logic. This way, we can prevent mix ups between
their mpiexecs and other potential matches, for example from an Intel
MPI installation.

This MR fixes a number of bugs stemming from legacy variable handling,
partially introduced by commit v3.10.0-rc1~103^2 (FindMPI: Add support
for FreeBSD and SLES, 2017-08-09).

* `MPI_COMPILE_OPTIONS` and `MPI_COMPILE_DEFINITIONS` legacy hints were not passed correctly.
* Legacy variables could be parsed after they were set by `FindMPI` cause erroneous hinting on subsequent runs.
* In the case of the compiler supporting MPI implicitly:
	* not all consumption variables would be correctly reset.
	* `MPI_<LANG>_HEADER_DIR` would be set unintentionally.
	* `MPI_<LANG>_LIB_NAMES` appeared in cache despite being ignored.
	* on subsequent passes, `MPI_<LANG>_COMPILER` could be corrupted.
* the C-to-CXX copy logic would take place even if `MPI_SKIP_GUESSING` was turned on
* if only some language compilers were given, inconsistent search behavior could be observed

Issue: #17538

Due to a erroneous logical dependency on a temporary variable, FindMPI
would depend its FOUND status on the set of flags needed to support
compiler-implicit MPI support after the first configure run.

This behavior is especially causing a misreported NOT FOUND status after
the first run on MPI implementations without MPI compiler like MS-MPI or
MPICH2 on Windows.

Due to a minor logic error, MPI_LIBRARY, MPI_EXTRA_LIBRARY and
MPI_<LANG>_LIBRARIES hints are currently not correctly handled and
discarded.

Additionally, if either C or CXX aren't enabled, there subsequent calls
to FindMPI will cause cache variables of the sort "MPI_MPI_EXTRA_LIBRARY-NOTFOUND"
to appear due to an error in the compatibility handling. (Doesn't affect
functionality)

If the compiler given in I_MPI_... could not be found, the Intel MPI
wrappers emit an error like "line 590: ifort: command not found".
The script should currently fail to match the output of this for
information, but we should generally treat such an output as invalid,
since the displayed configuration line can become a mixup between Intel
and GNU compiler settings.

Since commit v3.9.0-rc1~224^2~1 (FindMPI: MPIEXEC handling improvements,
2017-04-21) the `ProcessorCount` module is being used to initialize
`MPIEXEC_MAX_NUMPROCS`.  However, this leads to the logical cores being
counted rather than the physical ones, and some MPI implementations like
OpenMPI will error if mpiexec is called with that number.  Switch it to
the number of physical cores using `cmake_host_system_information`.

This ensures that if `MPIEXEC_MAX_NUMPROCS` is being used to set up MPI
tests with CTest or similar that the tests won't spuriously fail due to
OpenMPI refusing to start the application.

Fixes: #17117

This is a near-total rewrite of FindMPI implementing the following
features:

Environmental improvements:

- C++ settings can now be picked from C settings if no dedicated C++
  settings are found (some installs of IBM Platform MPI)
- Fortran settings can now be found on MPICH-1, MVAPICH-1, Microsoft
  MPI and MPICH-2 on Windows.

- Improved flag procession logic: Flags related to security enhancements
  of MPI itself, and unrelated to MPI usage (-fstack-protector-strong on
  e.g. RHEL 7 for MPICH and MVAPICH) will be filtered out properly now.
  -pthread is now being handled. -fno-strict-aliasing, which may be needed
  while compiling MVAPICH (e.g. RHEL 7's MVAPICH package) is also
  filtered.

Features added:

- MPI version can be determined for each language and is exposed as the
  package version
- MPI functionality is being tested to ensure the correctness of
  settings
- MPI-2 C++ bindings can be suppressed if desired by the project or
  user. For older MPIs, this often resolves issues and eliminated the
  infamous SEEK_SET trouble.
- MPI-2 C++ bindings are now being searched for and exposed by a
  variable if detected.
- Fortran support now does not make assumptions over the MPI present and
  works with ILP64 MPI implementations.
- Fortran now exposes which bindings (mpif.h, mpi/mpi_f08 modules) are
  available
- MPI detection can now work hybridly, which for example permits using the
  Fortran modules shipped with some compiler suites together with MPI
  (e.g. PGI on Windows shipping MSMPI modules)
- Ability to determine MPI library version (*requires* try_run, caution)
  upon request
- Ability to determine Fortran capabilities from MPI-3 for each of the
  bindings (subarrays, asynchronous attribute protecting entities) (also
  requires try_run)

Fine grained control over the search process:

- Possibility to selectively enable and disable steps like compiler
  wrapper search and guessing. Permits picking Microsoft MPI over Intel
  MPI if both are loaded (previously not possible without removing the
  IMPI compiler wrappers from the environment)
- Ability to pick MPICH2 over MSMPI (previously the first one located
  was taken)
- A user may now pass compiler flags to the compiler wrapper script,
  enabling much more flexibility. This permits for example linking the
  Intel MPI debug runtime, static linking, ILP64 with Fortran,
  linking tracing, ...
- A user may define a suffix for all MPI binaries that are being looked
  for (useful on Debian/Ubuntu where the alternative MPI installs are each
  appended with their name, e.g. mpicc.mpich and mpicc.lam)

Cache variable improvements

- Instead of the previous plural cache variables a list-of-variables
  approach is now in place. Permits correcting individual variables
  instead of having to wade through a list, also speeds up the search
  process and increases comfort by eliminating duplicate entries in
  LIBRARIES over languages
- MPIEXEC was corrected to MPIEXEC_EXECUTABLE in order to fix the
  grouping in the CMake GUI for the variable (previously ungrouped)

No need to explain this over and over again.  While at it, do some other
minor cleanups to whitespace and comments (i.e. delete them).

Per-source copyright/license notice headers that spell out copyright holder
names and years are hard to maintain and often out-of-date or plain wrong.
Precise contributor information is already maintained automatically by the
version control tool.  Ultimately it is the receiver of a file who is
responsible for determining its licensing status, and per-source notices are
merely a convenience.  Therefore it is simpler and more accurate for
each source to have a generic notice of the license name and references to
more detailed information on copyright holders and full license terms.

Our `Copyright.txt` file now contains a list of Contributors whose names
appeared source-level copyright notices.  It also references version control
history for more precise information.  Therefore we no longer need to spell
out the list of Contributors in each source file notice.

Replace CMake per-source copyright/license notice headers with a short
description of the license and links to `Copyright.txt` and online information
available from "https://cmake.org/licensing".  The online URL also handles
cases of modules being copied out of our source into other projects, so we
can drop our notices about replacing links with full license text.

Run the `Utilities/Scripts/filter-notices.bash` script to perform the majority
of the replacements mechanically.  Manually fix up shebang lines and trailing
newlines in a few files.  Manually update the notices in a few files that the
script does not handle.

Automate with:

find Modules -type f -print0 | xargs -0 perl -i -0pe \
's/set\(([a-zA-Z0-9_]+)(\s+)"\$\{\1\}([^"])/string(APPEND \1\2"\3/g'

The Intel MPI compiler wrappers link against static MPI libraries simply
by listing the libraries (no `-l`).

The Intel MPI wrappers use this form of -L to specify library locations.

Since commit v2.8.5~121^2~2 (FindMPI: Handle multiple languages,
2010-12-29) we called the GetPrerequisites is_file_executable function
but passed the name of a CMake variable instead of its value.  Therefore
the function has always failed and caused the search for the compiler
name to run even with an absolute path.  Switch to using if(IS_ABSOLUTE)
instead and drop use of GetPrerequisites.

Extend the workaround added by commit v3.2.0-rc1~278^2 (FindMPI:
Workaround Intel MPI 5.0.1 exit code problem, 2014-12-04) with an
additional/alternate keyword to recognize the case with recent GCCs.

When installing, MSMPI puts a trailing backslash in the MSMPI_BIN
environment variable. This causes trouble when concatenating in CMake
since the list separator is now escaped and no longer a list separator
due to the trailing backslash. Instead, use file(TO_CMAKE_PATH) to make
the path CMake-friendly.

Also comment what the various search paths are for.

Extend the regex that mathces -Wl, linker options to match -Xlinker too.
These are used by Intel MPI and perhaps others.

This MPI compiler may return zero even in some error cases.  Check the
output to catch such cases.
Suggested-by: Kelly Thompson <kgt@lanl.gov>

When testing CMAKE_<LANG>_COMPILER_ID values, do not explicitly
dereference or quote the variable. We want if() to auto-dereference the
variable and not its value. Also replace MATCHES with STREQUAL where
equivalent.

We already use MPI_HOME and ENV{MPI_HOME} as hints for the location
of the MPI compilers.  Do the same for mpiexec, and then use the
location of mpiexec as a hint to find the compilers.

The simple replacement is much faster.