Make assign build with GCC 4.4.
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.