Add version checks for compilers.
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

1cac803 Add the WriteCompilerDetectionHeader module.
23c7e19 Add a cxx_binary_literals feature.
422e754 Genex: Add $<HAVE_COMPILER_FEATURE:LANG,FEATURE> genex.
3c061ed Add a cxx_variadic_templates feature.
756c56a Remove version checks.
38a6d6f Add version checks for compilers.