build: use external gettext

To avoid a prartial redistribution of gettext, the macro AM_GNU_GETTEXT
has been configured with 'external' intlsymbol.

Change-Id: I59f64d78f2f4822a60e6abf3a1c5595cede8dbe7
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>