A common use case of `target_compile_features` is simply to specify that
the compiler should be run in a mode that is aware of e.g. C++11. Some
projects simply specify a particular C++11-only feature to request this.
Provide a first-class way to do this by naming features after the
corresponding language standard. Record them as always available in the
corresponding standard level so that requesting them always ensures that
standard (or higher) is used.
Since commit v3.1.0-rc1~635^2~7 (project: Add infrastructure for
recording CXX compiler features, 2013-10-17) we compile a test source to
a binary and then extract "<LANG>_FEATURES:..." strings from the binary
with the file(STRINGS) command. Add a newline at the beginning of the
string literal to be sure file(STRINGS) can extract the first entry as a
string independent of whatever else the compiler may put before the
storage it allocates for the literal within the binary.
Clang discards the entire string if it is not used, removing
the ability to read the features from the compiled binary. That
is prevented by using the symbol.
GNU with -O3 also discards the string, so use the string in a
way which is determined by a runtime value (argc) to prevent
it being discarded.
Add a feature test using the compiler macros and the preprocessor to
determine available features.
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.
This will be extended to other languages in the future. Follow-up
commits will add features which will be recorded by the feature test.
