Previously we weren’t checking for it in meson.build (but were checking
for it in configure.ac, courtesy of glib-gettext.m4). Roughly emulate
the checks from glib-gettext.m4, checking for bind_textdomain_codeset()
in whichever libintl implementation we found ngettext() in.

meson.build still doesn’t implement the full set and order of checks in
glib-gettext.m4; there’s still a FIXME about that in meson.build.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

#1313