找到SDL_ttf

找到 SDL_ttf 库

该模块定义：

SDL_TTF_LIBRARIES, the name of the library to link against
SDL_TTF_INCLUDE_DIRS, where to find the headers
SDL_TTF_FOUND, if false, do not try to link against
SDL_TTF_VERSION_STRING - human-readable string containing the version of SDL_ttf

为了向后兼容，还设置了以下变量：

SDLTTF_LIBRARY (same value as SDL_TTF_LIBRARIES)
SDLTTF_INCLUDE_DIR (same value as SDL_TTF_INCLUDE_DIRS)
SDLTTF_FOUND (same value as SDL_TTF_FOUND)

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