build: use external gettext

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

Change-Id: I22bab6fb8e4e3dbeb0f0ce5d513cfbb75b8a7402
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>