Run CxxDialect test only if compile options are available.
Merge topic 'target_compiler_features' into next

3dec32b Run CxxDialect test only if compile options are available.
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 target_compiler_features command.
Make COMPILER_FEATURES available as a target property.
Allow populating compiler_features using generator expressions.
Transitively evaluate compiler features.

Extend the interface of the target_compiler_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.
Add the WriteCompilerDetectionHeader module.

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

e1f25b0 Add the WriteCompilerDetectionHeader module.
bfa27f5 Transitively evaluate compiler features.
477e674 Allow populating compiler_features using generator expressions.
61d0e84 Make COMPILER_FEATURES available as a target property.
a330f27 Add target_compiler_features command.
cd411f4 Record the compilers capable of the cxx_delegating_constructors feature.
a7c44f4 Add CXX_STANDARD and CXX_EXTENSION target properties.