Autoconf m4 to check if minimum SDL library version exists

sdl_version_check.m4

# SDL
AC_ARG_ENABLE([sdl], AS_HELP_STRING([--enable-sdl], [Build SDL2 application]), [], [enable_sdl=check])
AS_IF([test "$enable_sdl" != "no"], [
    m4_include([m4/sdl2.m4])
    SDL_VERSION="2.0.5"
    AS_IF([test "$TRAVIS"], [SDL_VERSION=2.0.2])  # travis has old SDL, we don't care
    AS_IF([test "$EMSCRIPTEN"], [SDL_VERSION=2.0.0])  # emscripten has old SDL, we don't care

    # Check for libSDL >= $SDL_VERSION
    found_sdl=no
    AM_PATH_SDL2($SDL_VERSION,
        [found_sdl=yes; enable_sdl=yes],
        [:])

    AC_MSG_CHECKING(SDL2 >= $SDL_VERSION)
    AS_IF([test "$found_sdl" = "yes"], AC_MSG_RESULT([yes]), AC_MSG_RESULT(no))

    # blow up if user asked for SDL and we don't have it
    AS_IF([test "$enable_sdl" = "yes" && test "$found_sdl" = "no"], AC_MSG_ERROR([*** SDL version >= $SDL_VERSION not found!]))
])