FindSDL_net

找到 SDL_net 库

该模块定义

SDL_NET_LIBRARIES, the name of the library to link against
SDL_NET_INCLUDE_DIRS, where to find the headers
SDL_NET_FOUND, if false, do not try to link against
SDL_NET_VERSION_STRING - human-readable string containing the version of SDL_net

为了向后兼容，还设置了下列变量

SDLNET_LIBRARY (same value as SDL_NET_LIBRARIES)
SDLNET_INCLUDE_DIR (same value as SDL_NET_INCLUDE_DIRS)
SDLNET_FOUND (same value as SDL_NET_FOUND)

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