(CFLAGS): Do not include the WARNING_CFLAGS here, since GNU
Gettext doesn't compile cleanly, and dies with -Werror.
* src/Makefile.am, lib/Makefile.am, tests/atlocal.in (CFLAGS):
Include WARNING_CFLAGS here.
* lib/xstrdup.c: Include xalloc.h, so that xstrdup be declared
before being defined.