build: Use more gnulib modules.

* autogen.sh (GNULIB_MODULES): Add getopt-gnu.
Use directory gnulib-local.
* gnulib-local/lib/getopt-pfx-ext.h.diff: New file.
* Makefile.in (SOURCE_FILES): Add it.
(distdir): Use 'mkdir -p' instead of 'mkdir'.
* lib/getopt.h: Remove file.
* lib/getopt.c: Remove file.
* lib/getopt1.c: Remove file.
* lib/Makefile.am (libgp_a_SOURCES): Remove them.

lib/.gitignore

@@ -22,11 +22,22 @@
 /ftell.c
 /ftello.c
 /getdtablesize.c
+/getopt-cdefs.in.h
+/getopt-core.h
+/getopt-ext.h
+/getopt-pfx-core.h
+/getopt-pfx-ext.h
+/getopt.c
+/getopt.in.h
+/getopt1.c
+/getopt_int.h
+/gettext.h
 /idx.h
 /intprops-internal.h
 /inttypes.in.h
 /limits.in.h
 /lseek.c
+/locale.in.h
 /malloc.c
 /malloca.c
 /malloca.h