gnulib: let it use its own PO domain

See
https://www.gnu.org/software/gnulib/manual/html_node/Localization.html.

* bootstrap.conf: Create gnulib-po.
* Makefile.am, configure.ac: Use it.
* po/POTFILES.in: Remove files now in gnulib.
* src/main.c: Open the bison-gnulib domain.

po/POTFILES.in

@@ -17,15 +17,3 @@ src/scan-code.l
 src/scan-gram.l
 src/scan-skel.l
 src/symtab.c
-
-lib/argmatch.c
-lib/closeout.c
-lib/error.c
-lib/getopt.c
-lib/obstack.c
-lib/spawn-pipe.c
-lib/quotearg.c
-lib/timevar.c
-lib/w32spawn.h
-lib/wait-process.c
-lib/xalloc-die.c