Make 'configure' a bit smaller and faster.

Make 'configure' a bit smaller and faster.

* configure.in (INSTALL_INFO): Set it with one call to
AC_PATH_PROG, not three.
(PKG_CONFIG): Hoist this out of PKG_CHECK_MODULES, since it's
so likely that it'll be needed.
(AC_CHECK_HEADERS_ONCE): Prefer to AC_CHECK_HEADERS where either works.
(AC_PROG_MAKE_SET): Remove; Automake does this.
(sysinfo): Do not check for this function, as it's never used.
(tzset): Check for this function just once.
* admin/CPP-DEFINES: Remove HAVE_SYSINFO.
* m4/manywarnings.m4: Sync from gnulib, incorporating the following:
2012-05-27 manywarnings: remove duplicate -Wmultichar entry