dynarray: Avoid conflict with preprocessor macros owned by the system.
* lib/dynarray.h: Don't include <libc-config.h>. Define _GL_LIKELY,
_GL_UNLIKELY. Include malloc/dynarray.gl.h instead of malloc/dynarray.h.
Include malloc/dynarray-skeleton.gl.h instead of
malloc/dynarray-skeleton.c.
* modules/dynarray (Depends-on): Add builtin-expect.
(Makefile.am): Arrange to create malloc/dynarray.gl.h from
malloc/dynarray.h and malloc/dynarray-skeleton.gl.h from
malloc/dynarray-skeleton.c.