Fixed building config.c on NetBSD

Hi pev developers,
config.c does not build at the moment on NetBSD. It makes use of the "PATH_MAX" constant, which according to POSIX should be available from <limits.h>. Another version of this fix would therefore be to unconditionally include this header instead - I simply cannot tell if it really is available on every platform you support at the moment.
Generally wise, I believe it is best to not rely on PATH_MAX to allocate fixed-size buffers for filenames anyway, because some systems do not have any such limitation in the first place.
HTH,
-- khorben