libc-config: port to DragonFlyBSD 5.9 DragonFlyBSD defines __nonnull incompatibly with glibc, so avoid the use of __nonnull in Gnulib code. Problem reported by Gavin Smith in: https://lists.gnu.org/r/bug-gnulib/2021-03/msg00066.html * lib/cdefs.h (__attribute_nonnull__): Rename from __nonnull. All uses in Gnulib changed. There should be no need to change glibc code that is not shared with Gnulib. (__nonnull): New macro, defined in terms of __attribute_nonnull__, and which can be used in glibc code that is not shared with Gnulib.