build: use more gnulib modules for simpler code

build: use more gnulib modules for simpler code

* .gnulib: Update to latest, for sigpipe and sigaction modules.
* bootstrap.conf (gnulib_modules): Add siaction, sigpipe, strerror_r.
* tools/virsh.c (vshSetupSignals) [!SIGPIPE]: Delete, now that
gnulib guarantees it.
(SA_SIGINFO): Define for mingw fallback.
* src/util/virterror.c (virStrerror): Simplify, now that gnulib
guarantees the POSIX interface.
* configure.ac (AC_CHECK_FUNCS_ONCE): Drop redundant check.
(AM_PROG_CC_STDC): Move earlier, to keep autoconf happy.