xalloc-die: Fix link error with Solaris cc (regression 2020-07-27). * lib/xalloc.h (xalloc_die): Don't declare if GNULIB_XALLOC_DIE is 0. (xmalloc, xzalloc, xcalloc, xrealloc, x2realloc, xmemdup, xstrdup, XMALLOC, XNMALLOC, XZALLOC, XCALLOC, xnmalloc, xnrealloc, x2nrealloc, xcharalloc): Don't declare/define if GNULIB_XALLOC is 0. * modules/xalloc (configure.ac): Define GNULIB_XALLOC. * modules/xalloc-die (configure.ac): Define GNULIB_XALLOC_DIE.