libc-config: Avoid overriding the headers from an installed newer glibc. Reported by Paul E Murphy <murphyp@linux.ibm.com> in <https://lists.gnu.org/archive/html/bug-gnulib/2020-12/msg00106.html>. * lib/cdefs.h: Use a different include guard than glibc's <sys/cdefs.h>. * lib/libc-config.h: Update accordingly.