Fix matching of languages in the LINGUAS environment variable.
Setting LINGUAS e.g. to "frs" should not cause fr.mo to get installed,
because Frisian is not a variant of French.
Reported by <arfrever.fta@gmail.com> in <https://savannah.gnu.org/bugs/?59062>.
* gettext-runtime/m4/po.m4 (AM_PO_SUBDIRS, AM_POSTPROCESS_PO_MAKEFILE): Don't
consider $desiredlang a variant of $presentlang unless it has the syntax of a
variant.
