Makefile: add GNU_GETTEXT, set when we expect GNU gettext

Makefile: add GNU_GETTEXT, set when we expect GNU gettext

Add a GNU_GETTEXT flag that's set to YesPlease by default everywhere
but on Solaris if the NO_GETTEXT flag isn't set.

Everyone uses GNU gettext these days except Solaris developers, so
having a dumb check for this that just depends on SunOS should be
OK.

I also don't plan to use this (for the time being) for anything that
would break if it isn't correctly set (e.g. if you're using GNU
libintl on Solaris), so I haven't added paranoid autoconf guards.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>