This, of course, …
checking foo.h usability... yes checking foo.h presence... no configure: WARNING: foo.h: accepted by the compiler, rejected by the preprocessor! configure: WARNING: foo.h: proceeding with the compiler's result
… happens when your configure.in sets CFLAGS where it ought to have set CPPFLAGS.
Sigh. I’m currently hunting a bug in MPlayer which causes the program
to crash on SIGFPE when trying to play FLAC or MP3. I have quite a few
one-line fixes for MPlayer. Kinda pisses me off. I feel for you.