CMake: Only set CMAKE_CXX_FLAGS after enable_language(CXX)
CMake: Only set CMAKE_CXX_FLAGS after enable_language(CXX)

Otherwise flags specified in a CMake cross-compilation toolchain (such as
CMAKE_CXX_FLAGS_INIT) will not be added. This could also affect native
builds but is much more likely to be noticed when cross-compiling due to
missing essential flags.

libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat