cmake OPNMIDI_HAS_SELECT_SONG_NUM examples

WohlSoft/SDL-Mixer-X src/codecs/music_midi_opn.cmake :39

try_compile(OPNMIDI_HAS_SELECT_SONG_NUM
                ${CMAKE_BINARY_DIR}/compile_tests
                ${SDLMixerX_SOURCE_DIR}/cmake/tests/opnmidi_get_songs_count.c
                LINK_LIBRARIES ${OPNMIDI_LIBRARIES} ${STDCPP_LIBRARY} ${M_LIBRARY}
                CMAKE_FLAGS "-DINCLUDE_DIRECTORIES=${OPNMIDI_INCLUDE_DIRS}"
                OUTPUT_VARIABLE OPNMIDI_HAS_SELECT_SONG_NUM_RESULT
            )