gnulib: delete all gnulib integration

gnulib: delete all gnulib integration

This deletes all trace of gnulib from libvirt. We still
have the keycodemapdb submodule to deal with. The simple
solution taken was to update it when running autogen.sh.

Previously gnulib could auto-trigger refresh when running
'make' too. We could figure out a solution for this, but
with the pending meson rewrite it isn't worth worrying
about, given how infrequently keycodemapdb changes.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>