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.
Remove version checks.

These files can't have version checks as they can be read by a
CMake which has a different compiler loaded.
Add a cxx_variadic_templates feature.
Genex: Add $<HAVE_COMPILER_FEATURE:LANG,FEATURE> genex.
Add a cxx_binary_literals feature.

Supported by GCC in c++98 mode, though it is a c++1y standard feature.
Add the WriteCompilerDetectionHeader module.

Provide a function to write a portable header to detect compiler
features.
Merge topic 'target_compile_features' into next

c8605c9 Add the WriteCompilerDetectionHeader module.
531620c Add a cxx_binary_literals feature.
4bd4da6 Genex: Add $<HAVE_COMPILER_FEATURE:LANG,FEATURE> genex.
0acfb5c Add a cxx_variadic_templates feature.
f2c14bd Remove version checks.
8bbbee3 Base the target_compile_features command on cmTargetPropCommandBase.
f84c532 cmTargetPropCommandBase: Change the interface to return bool.
91cdb7c cmTarget: Transitively evaluate compiler features.
d438d5b cmTarget: Allow populating COMPILE_FEATURES using generator expressions.
11e3ed6 cmTarget: Make COMPILE_FEATURES available as a target property.
9a62941 Add target_compile_features command.
a7b356b Record compilers capable of msvcxx_sealed feature.
90ab0f0 Record compilers capable of the gnuxx_typeof feature.
aea693e Record the compilers capable of the cxx_delegating_constructors feature.
89e36df cmTarget: Add CXX_STANDARD and CXX_EXTENSION target properties.