package/Makefile.in: correct gettext handling for musl

Based on a patch by Bernd Kuhls.

The AM_GNU_GETTEXT autotools macro misdetects musl gettext support as it
checks for internal glibc symbols.  Work around it by forcing libc gettext
support when musl is used for the supported gettext api levels.

As this is a generic issue for any package using AM_GNU_GETTEXT, add it to
the global TARGET_CONFIGURE_ARGS instead of for each affected package.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>