cmake HAVE_ATTRIBUTE_SYMVER examples

mmontag/chip-player-js libxmp/cmake/libxmp-checks.cmake :183 

check_c_source_compiles("void foo(void) __attribute__((__symver__(\"foo@bar\")));
                             int main(void) { return 0; }" HAVE_ATTRIBUTE_SYMVER)

mmontag/chip-player-js libxmp/cmake/libxmp-checks.cmake :199 

list(APPEND LIBXMP_DEFINES -DHAVE_ATTRIBUTE_SYMVER)