AC_SEARCH_LIBS will give a result of "none required" if the relevant
 function is available without any special libraries.
This is the case for lrintf on MinGW.
Make sure this special value isn't put verbatim into the pkg-config
 files, as it would cause the linker to search for files named
 "none" and "required", and fail.

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>