Help: Reformat the generator expression documentation.

Use description lists instead of a preformatted block to enumerate
the genex documentation.

Consistently capitalize the description.

Use ``...`` to format genex parameters in the description.

Turn references to commands into links.

Add high level documentation about the motivation for generator
expressions.

Regroup expressions into subsections for different genex types. Add
a high-level description and example of the expressions in each
section.

Explode the documentation for filesystem artifacts, instead of only
referring to the variations.
Merge topic 'genex-docs' into next

fa59107 Help: Reformat the generator expression documentation.
Don't load Clang-CXX from AppleClang-CXX.

The Clang-CXX module is going to get version-specific checks, and
the version system for AppleClang is not the same as Clang.
Add some COMPILE_OPTIONS for specifying C++ dialect.

These are compiler-specific, compiler version specific, extension specific
and standard version specific.
cmTarget: Add CXX_STANDARD and CXX_EXTENSION target properties.

These are used to determine whether to add -std=c++11, -std=gnu++11
etc flags on the compile line.
Record the compilers capable of the cxx_delegating_constructors feature.

Add a feature test using the compiler macros and the preprocessor to
determine available features. Add a unit test which outputs the
feature test file compiled. Record the list of features in
the SystemInformation unit test.

Add a CMAKE_CXX_COMPILE_FEATURES variable which contains all features
known to the loaded compiler, and a CMAKE_CXX_KNOWN_FEATURES variable
containing all features known to CMake. Add language standard specific
variables for internal use to determine the standard-specific compile
flags to use. Add internal variables matching CMAKE_PP_NAME_<feature>
for each feature. Those variables are used as preprocessor defines.
Record compilers capable of the gnuxx_typeof feature.
Record compilers capable of msvcxx_sealed feature.

Clang-cl is capable of this feature, but that is not yet released,
nor have I tested it.
Add target_compile_features command.

This can be used to set the compiler features required by particular
targets. An error is issued at CMake time if the compiler does not
support the required feature. If a language dialect flag is required
by the features used, that will be added automatically.
cmTarget: Make COMPILE_FEATURES available as a target property.
cmTarget: Allow populating COMPILE_FEATURES using generator expressions.
cmTarget: Transitively evaluate compiler features.

Extend the interface of the target_compile_features command with
PUBLIC and INTERFACE keywords. Populate the INTERFACE_COMPILER_FEATURES
target property if they are set. Consume the INTERFACE_COMPILER_FEATURES
target property from linked dependent targets to determine the final
required compiler features and the compile flag, if needed.
cmTargetPropCommandBase: Change the interface to return bool.

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

This gives us 'free' handling of IMPORTED, ALIAS, INTERFACE,
non-compilable and missing targets.
Merge topic 'target_compile_features' into next

de1427a Base the target_compile_features command on cmTargetPropCommandBase.
c4a993f cmTargetPropCommandBase: Change the interface to return bool.
deae4c1 cmTarget: Transitively evaluate compiler features.
f8b1eb8 cmTarget: Allow populating COMPILE_FEATURES using generator expressions.
82d03ea cmTarget: Make COMPILE_FEATURES available as a target property.
ddabfa8 Add target_compile_features command.
398c770 Record compilers capable of msvcxx_sealed feature.
a507a71 Record compilers capable of the gnuxx_typeof feature.
220e97f Record the compilers capable of the cxx_delegating_constructors feature.
67d4930 cmTarget: Add CXX_STANDARD and CXX_EXTENSION target properties.
d07fb52 Add some COMPILE_OPTIONS for specifying C++ dialect.
1c41914 Don't load Clang-CXX from AppleClang-CXX.