程式語言 - Simple DirectMedia Layer (SDL) - v2.0 - 解決Function無法Export的問題

參考資訊：
https://gcc.gnu.org/wiki/Visibility

CheckVisibilityHidden()被用來設定-fvisibility=hidden，因此，需要將Makefile檔案中的-fvisibility=hidden改成-fvisibility=default

CheckVisibilityHidden()
{
    AC_MSG_CHECKING(for GCC -fvisibility=hidden option)
    have_gcc_fvisibility=no

    visibility_CFLAGS="-fvisibility=hidden"
    save_CFLAGS="$CFLAGS"
    CFLAGS="$save_CFLAGS $visibility_CFLAGS -Werror"
    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
    #if !defined(__GNUC__) || __GNUC__ < 4
    #error SDL only uses visibility attributes in GCC 4 or newer
    #endif
    ]],[])], [have_gcc_fvisibility=yes],[])
    AC_MSG_RESULT($have_gcc_fvisibility)
    CFLAGS="$save_CFLAGS"

    if test x$have_gcc_fvisibility = xyes; then 
        EXTRA_CFLAGS="$EXTRA_CFLAGS $visibility_CFLAGS"
    fi
}