build: Remove gnulib.

As part of our efforts to clean up and simplify libguestfs, removing
gnulib deletes a large dependency that we mostly no longer use and
causes problems for new users trying to build the library from source.

A few modules from gnulib are still used (under a compatible license)
and these are copied into gnulib/lib/

fuse/guestmount.c

@@ -30,6 +30,7 @@
 #include <error.h>
 #include <getopt.h>
 #include <signal.h>
+#include <limits.h>
 #include <locale.h>
 #include <libintl.h>