Fix 'attribute declaration must precede definition' warning (clang-3.1)

* extra/gc.c: Include gc_inline.h explicitly before including mallocx.c
(to workaround "attribute declaration must precede definition" warning
reported by clang-3.1 for GC_generic_malloc_many); define
GC_PTHREAD_REDIRECTS_ONLY before including gc_pthread_redirects.h.
* include/gc_pthread_redirects.h (GC_dlopen, GC_pthread_sigmask,
GC_PTHREAD_CREATE_CONST, GC_pthread_create, GC_pthread_join,
GC_pthread_detach, GC_pthread_cancel, GC_pthread_exit): Do not declare
if GC_PTHREAD_REDIRECTS_ONLY defined (to workaround
"attribute declaration must precede definition" warning reported by
clang-3.1).