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names and years are hard to maintain and often out-of-date or plain wrong.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

Modern editors provide plenty of ways to visually separate functions.
Drop the explicit comments that previously served this purpose.
Use the following command to automate the change:

    $ git ls-files -z -- \
        "*.c" "*.cc" "*.cpp" "*.cxx" "*.h" "*.hh" "*.hpp" "*.hxx" |
      egrep -z -v "^Source/cmCommandArgumentLexer\." |
      egrep -z -v "^Source/cmCommandArgumentParser(\.y|\.cxx|Tokens\.h)" |
      egrep -z -v "^Source/cmDependsJavaLexer\." |
      egrep -z -v "^Source/cmDependsJavaParser(\.y|\.cxx|Tokens\.h)" |
      egrep -z -v "^Source/cmExprLexer\." |
      egrep -z -v "^Source/cmExprParser(\.y|\.cxx|Tokens\.h)" |
      egrep -z -v "^Source/cmFortranLexer\." |
      egrep -z -v "^Source/cmFortranParser(\.y|\.cxx|Tokens\.h)" |
      egrep -z -v "^Source/cmListFileLexer\." |
      egrep -z -v "^Source/cm_sha2" |
      egrep -z -v "^Source/(kwsys|CursesDialog/form)/" |
      egrep -z -v "^Utilities/(KW|cm).*/" |
      xargs -0 sed -i '/^\(\/\/---*\|\/\*---*\*\/\)$/ {d;}'

This avoids modifying third-party sources and generated sources.

All compilers hosting CMake support the std class.

This allows backtraces to be fully controlled by the makefile rather
than externally (and makes changing how they are manipulated easier).

This is needed for the target_compile_features command, which
may fail at configure time if an invalid feature is specified.

This is similar to the include_directories(SYSTEM) signature
in that it allows telling the compiler to ignore warnings from
such headers.

This command populates the COMPILE_OPTIONS target property.

This way we can add handling of relative/absolute paths and of
-D in compile definitions.

With similar reasoning to the parent commit, as downstreams, we can't
determine what $<CONFIG> generator expressions would be appropriate.

Upstream would have populated the INTERFACE_INCLUDE_DIRECTORIES with
config-specific generator expressions, possibly appropriate for
their DEBUG_CONFIGURATIONS. In theory, if we would add include
directories for a DEBUG intent, we would have to match the upstream
configurations for that.

Rather than attempting to discover the appropriate configurations
at this time, simplify the feature instead. The use of IMPORTED targets
with these commands could still be added in the future if targets
would export their DEBUG_CONFIGURATIONS somehow.

This is a convenience API to populate the corresponding properties.