找到SDL_mixer

找到 SDL_mixer 库

该模块定义：

SDL_MIXER_LIBRARIES, the name of the library to link against
SDL_MIXER_INCLUDE_DIRS, where to find the headers
SDL_MIXER_FOUND, if false, do not try to link against
SDL_MIXER_VERSION_STRING - human-readable string containing the
                           version of SDL_mixer

为了向后兼容，还设置了以下变量：

SDLMIXER_LIBRARY (same value as SDL_MIXER_LIBRARIES)
SDLMIXER_INCLUDE_DIR (same value as SDL_MIXER_INCLUDE_DIRS)
SDLMIXER_FOUND (same value as SDL_MIXER_FOUND)

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