freebsd: Fix build problem due to picking up the wrong libvirt.h

freebsd: Fix build problem due to picking up the wrong libvirt.h

Gettext annoyingly modifies CPPFLAGS in-place, putting
-I/usr/local/include into the search patch if libintl headers
must be used from that location.  But since we must support
automake 1.9.6 which lacks AM_CPPFLAGS, and since CPPFLAGS is used
prior to INCLUDES, this means that the build picks up the _old_
installed libvirt.h in priority to the in-tree version, leading
to all sorts of weird build failures on FreeBSD.

Fix this by teaching configure to undo gettext's actions, but
to keep any changes required by gettext at the end of INCLUDES
after all in-tree locations are used first.  Also requires
adding a wrapper Makefile.am and making gnulib-tool create
just gnulib.mk files during the bootstrap process.

Signed-off-by: Eric Blake <eblake@redhat.com>