postgresql*-client: fix installed PGXS Makefiles when using pkgsrc gettext

If we are using the "nls" PKG_OPTION together with pkgsrc gettext, then
we must ensure CPPFLAGS contains -I${PREFIX}/include/gettext.
The installed postgresql/server/c.h expects to be able to include
<libintl.h> using the CPPFLAGS that get installed in Makefile.global.
Without this it is not possible to develop PGXS extensions
outside of pkgsrc.

(tnn)