Change builtin features good enough to replace GNU gettext from ngettext()
to dcgettext(), because BUILDLINK_API_DEPENDS.gettext-lib is now >=0.18.
* bump required gettext-tools from pkgsrc to 0.15 to match required gettext-lib   capability.
* prevent to restrict gettext-tools capability from builtin gettext-tools' one
  even if using it from pkgsrc.