Remove direct compiler invocation for feature testing.

A single code path for this is a lesser maintenance burden.
Merge topic 'target_compile_features' into next

fbcf62d Remove direct compiler invocation for feature testing.
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

b6eae03 Base the target_compile_features command on cmTargetPropCommandBase.
1f44cb0 cmTargetPropCommandBase: Change the interface to return bool.
9b3ffdc cmTarget: Transitively evaluate compiler features.
d259473 cmTarget: Allow populating COMPILE_FEATURES using generator expressions.
dbf85dc cmTarget: Make COMPILE_FEATURES available as a target property.
465c230 Add target_compile_features command.
7c05333 Record compilers capable of msvcxx_sealed feature.
1737921 Record compilers capable of the gnuxx_typeof feature.
0cd8c86 Record the compilers capable of the cxx_delegating_constructors feature.