The main change is to add a NEWER_GCC macro, separate from NEW_GCC, to
detect GCC 4.3 and up.

This is necessary because, in GCC 4.3, hash_fun.h moved from ext/ to
backward/.  New comments in port.h give the details.

common.h, fstring.h: use NEWER_GCC to locate hash_fun.h.  port.h:
define NEWER_GCC if GCC >= 4.3.