The COMPILE_LANGUAGE generator expression is only supported since CMake
3.3. Moreover, it does not work with all generators (works with Makefile
and Ninja, but not with Visual Studio).

target_compile_options would only work if a target does not mix C and
C++ sources, since the flags are intended to be set for a specific
language, use set_source_files_properties instead. This approach is also
less repetitive.

Drop the idea of using lists and COMPILE_OPTIONS,
set_source_files_properties only understands COMPILE_FLAGS (a single
string, not a list).