Fix port to 32-bit AIX with xlc.

Fix port to 32-bit AIX with xlc.

This doesn't fix Bug#17598, but it does fix a regression since Emacs
built with xlc until recently, and perhaps it'll help us fix Bug#17598.
* configure.ac (GC_SETJMP_WORKS): Define for AIX, too.
Merge from gnulib, incorporating:
2014-05-30 ftoastr: work around compiler bug in IBM xlc 12.1
* lib/ftoastr.c: Update from gnulib.
* src/alloc.c (gdb_make_enums_visible): Remove FLOAT_TO_STRING_BUFSIZE.
* src/conf_post.h (FLEXIBLE_ARRAY_MEMBER) [__IBMC__]: Don't define to empty.
* src/lisp.h (FLOAT_TO_STRING_BUFSIZE): Make it a macro, instead of an enum,
to work around a compiler bug in IBM xlc 12.1.