找到SDL_gfx

在 3.25 版本加入.

找到 SDL_gfx 库

该模块定义：

SDL::SDL_gfx, the name of the target to use with target_*() commands
SDL_GFX_LIBRARIES, the name of the library to link against
SDL_GFX_INCLUDE_DIRS, where to find the headers
SDL_GFX_FOUND, if false, do not try to link against
SDL_GFX_VERSION_STRING - human-readable string containing the
                           version of SDL_gfx

$SDLDIR 是一个环境变量，对应于构建 SDL 时使用的 ./configure --prefix=$SDLDIR。