cmake: Separate debug symbols on gcc

gistfile1.txt

# https://git.magicalgirl.moe/STJr/SRB2/commit/e08bd452279dbeedbf876fec465ea51a137455a8

## strip debug symbols into separate file when using gcc
if(CMAKE_COMPILER_IS_GNUCC)
    if(${CMAKE_BUILD_TYPE} MATCHES Debug)
        message(STATUS "Will make separate debug symbols in *.debug")
        add_custom_command(TARGET ${SRB2_SDL2_EXE_NAME} POST_BUILD
            COMMAND ${OBJCOPY} --only-keep-debug $<TARGET_FILE:${SRB2_SDL2_EXE_NAME}> $<TARGET_FILE:${SRB2_SDL2_EXE_NAME}>.debug
            COMMAND ${OBJCOPY} --strip-debug $<TARGET_FILE:${SRB2_SDL2_EXE_NAME}>
            COMMAND ${OBJCOPY} --add-gnu-debuglink=$<TARGET_FILE_NAME:${SRB2_SDL2_EXE_NAME}>.debug $<TARGET_FILE:${SRB2_SDL2_EXE_NAME}>
        )
    endif()
endif()