fpending: fix regression on DragonFly BSD Commit 3f5bf99e claimed that it is safe to declare __fpending twice, but in reality this caused a regression on Dragonfly BSD. Reported by GW in <https://lists.gnu.org/archive/html/bug-m4/2013-11/msg00000.html>, <https://lists.gnu.org/archive/html/bug-gnulib/2013-11/msg00022.html>. * m4/fpending.m4 (gl_FUNC_FPENDING): Check for declaration. * lib/fpending.h (__fpending): Don't declare twice. Signed-off-by: Eric Blake <eblake@redhat.com>