While at it change some single character additions to be of type char.

This changes `cmMakefile::AddDefinition` to take a `cm::string_view` as value
argument instead of a `const char *`.

Benefits are:
- `std::string` can be passed to `cmMakefile::AddDefinition` directly without
  the `c_str()` plus string length recomputation fallback.
- Lengths of literals passed to `cmMakefile::AddDefinition` can be computed at
  compile time.

In various sources uses of `cmMakefile::AddDefinition` are adapted to avoid
`std::string::c_str` calls and the `std::string` is passed directly.
Uses of `cmMakefile::AddDefinition`, where a `nullptr` `const char*` might
be passed to `cmMakefile::AddDefinition` are extended with `nullptr` checks.

Replaced most manual `const_iterator`-based loops and some
reverse-iterator loops with range loops.

Fixes: #18858

Reduce the number of files relying on `cmake.h`.

* Change some functions to take `std::string` instead of
  `const char*` in the following classes: `cmMakeFile`, `cmake`,
  `cmCoreTryCompile`, `cmSystemTools`,  `cmState`, `cmLocalGenerator`
  and a few others.
* Greatly reduce using of `const char*` overloads for
  `cmSystemTools::MakeDirectory` and `cmSystemTools::RelativePath`.
* Remove many redundant `c_str()` conversions throughout the code.

This commit continues the changes made in CTest to support std::chrono
by
applying it throughout every component where a duration was used.

No functional change intended.

Automate with:

git grep -l '#include <cm_' -- Source \
  | xargs sed -i 's/#include <\(cm_.*\)>/#include "\1"/g'

git grep -l '#include <cmsys/' -- Source \
  | xargs sed -i 's/#include <\(cmsys\/.*\)>/#include "\1"/g'

git grep -l '#include <cm[A-Z]' -- Source \
  | xargs sed -i 's/#include <\(cm[A-Z].*\)>/#include "\1"/g'

Port dependent code to the change.

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names and years are hard to maintain and often out-of-date or plain wrong.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.

Run the `Utilities/Scripts/clang-format.bash` script to update
all our C++ code to a new style defined by `.clang-format`.
Use `clang-format` version 3.8.

* If you reached this commit for a line in `git blame`, re-run the blame
  operation starting at the parent of this commit to see older history
  for the content.

* See the parent commit for instructions to rebase a change across this
  style transition commit.

The clang-format tool can do a good job formatting most code, but
well-organized streaming blocks are best left manually formatted.

Find blocks of the form

    os <<
      "...\n"
      "...\n"
      ;

using the command

    $ git ls-files -z -- Source |
      egrep -v -z '^Source/kwsys/' |
      xargs -0 pcregrep -M --color=always -B 1 -A 1 -n \
        '<<[^\n]*\n(^ *("[^\n]*("|<<|;)$|;)\n){2,}'

Find blocks of the form

    os << "...\n"
       << "...\n"
       << "...\n";

using the command

    $ git ls-files -z -- Source |
      egrep -v -z '^Source/kwsys/' |
      xargs -0 pcregrep -M --color=always -B 1 -A 1 -n \
        '<<[^\n]*\n(^ *<<[^\n]*(\\n"|<<|;)$\n){2,}'

Surround such blocks with the pair

    /* clang-format off */
    ...
    /* clang-format on */

in order to protect them from update by clang-format.  Use the C-style
`/*...*/` comments instead of C++-style `//...` comments in order to
prevent them from ever being swallowed by re-formatting of surrounding
comments.

Each source file has a logical first include file.  Include it in an
isolated block so that tools that sort includes do not move them.

Create a `CMAKE_TRY_COMPILE_TARGET_TYPE` option to specify use
of `add_library(... STATIC ...)` for the generated test project.
This will be useful for cross-compiling toolchains that cannot
link a binary without custom flags or scripts.

Allow the name to be used for something more-suitable.

Extend the RunSingleCommand signature to capture stdout and stderr
separately.  Allow both to be captured to the same std::string
to preserve existing behavior.  Update all call sites to do this
so that this refactoring does not introduce functional changes.

This simplifies reasoning about the follow-up commit which ports
away from cmCacheManager to a class with the same method names.

If the CMAKE_CROSSCOMPILING_EMULATOR variable is defined, and
CMAKE_CROSSCOMPILING is TRUE, then use CMAKE_CROSSCOMPILING_EMULATOR to run
the try_run executables. This prevents the need to populate
TryRunResults.cmake when cross compiling.

This topic was never tested without some follow-up commits.  The
GetCacheEntryValue API returns a pointer to memory freed on return.
It will have to be revised along with the rest of the original topic.

This simplifies reasoning about the follow-up commit which ports
away from cmCacheManager to a class with the same method names.

Most functionality is already implemented in Source/cmCoreTryCompile.{h,cxx}.
Document and improve argument parsing.

This functionality is already being used by a number of modules, like
CheckCSourceCompiles.cmake, but it is not documented.