Use nominative/genitive month names correctly
These changes are needed since the standalone (nominative) vs. full date format (genitive) month names support has been introduced in Linux with glibc 2.27. They are also supported in *BSD systems.
See also: https://bugzilla.gnome.org/show_bug.cgi?id=749206 https://sourceware.org/bugzilla/show_bug.cgi?id=10871
Closes: #125 (closed)