* lisp.h (USE_STACK_STRING): Now true only if USE_STACK CONS.

On x86 platforms this works around GCC bug 63495
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63495>,
and more generally should fix a portability problem in Emacs.
Problem reported by Stefan Monnier in:
http://lists.gnu.org/archive/html/emacs-devel/2014-10/msg00261.html