gen-options.awk: Fix test with older gettext.

gen-options.awk: Fix test with older gettext.

Really old versions of xgettext do not support msgctx, but they do
produce _some_ output so the test is not skipped, just fails.

Enhance the xgettext probe to skip the test with these old versions, and
split it into two so we can still get a proper pass result for the rest.