FindSDL_image

找到 SDL_image 库

此模块定义

SDL_IMAGE_LIBRARIES, the name of the library to link against
SDL_IMAGE_INCLUDE_DIRS, where to find the headers
SDL_IMAGE_FOUND, if false, do not try to link against
SDL_IMAGE_VERSION_STRING - human-readable string containing the
                           version of SDL_image

为了向后兼容，还设置了以下变量

SDLIMAGE_LIBRARY (same value as SDL_IMAGE_LIBRARIES)
SDLIMAGE_INCLUDE_DIR (same value as SDL_IMAGE_INCLUDE_DIRS)
SDLIMAGE_FOUND (same value as SDL_IMAGE_FOUND)

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