Similar to https://gitlab.gnome.org/GNOME/evolution/issues/196
when intltool-merge is called in parallel, it could either rewrite
the ongoing attempt to build it or use an incomplete data, which
results in broken output files (.desktop, .metainfo and so on).
This change ensures the intltool-merge cache is created only once
and any other requests which would use it will wait until it's created.
