package/libgpg-error: optionally depends on gettext

configure.ac has AM_GNU_GETTEXT(), which will enable i18n if a gettext
library is found. For uClibc, it is found if the gettext package has
been built, and it will add -lintl to the link flags. For musl and
glibc, it is always found, in libc itself so nothing is added to the
link flags.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Matthew Weber <matt@thewebers.ws>
Tested-by: Matthew Weber <matt@thewebers.ws>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>