build: prohibit cross-inclusion

build: prohibit cross-inclusion

Make it easier to detect invalid cross-directory includes, by
adding a syntax check.  The check is designed to be extensible:
the default case lists only the non-driver directories, and
specific directories can list a different set (for example,
util/ can only use itself, network/ can only use itself, util/,
or conf/).

* .gnulib: Update to latest, for syntax check improvment.
* cfg.mk (sc_prohibit_cross_inclusion): New check.
(sc_prohibit_strncmp, sc_libvirt_unmarked_diagnostics): Simplify.