cmake OPNMIDI_HAS_CHANNEL_ALLOC_MODE_RESULT examples

WohlSoft/SDL-Mixer-X src/codecs/music_midi_opn.cmake :22 

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