po: merge translations for metainfo file

When installing the module, instead of copying the metainfo file to the
destination, merge the available translations for it. Since this is done
at install time, add gettext as BuildRequires for the RPM packaging;
add libappstream-glib-devel as well only on RHEL/CentOS 8, since it
contains the metainfo.its for gettext in order to handle .metainfo.xml
files.

This requires a LINGUAS file with the list of translations currently
available: this is the way msgfmt picks the available languages to
merge. Generate it dynamically from the list of available .po files.

Makefile

@@ -46,6 +46,9 @@ po/$(PACKAGE_NAME).metainfo.pot: org.cockpit-project.$(PACKAGE_NAME).metainfo.xm
 po/$(PACKAGE_NAME).pot: po/$(PACKAGE_NAME).html.pot po/$(PACKAGE_NAME).js.pot po/$(PACKAGE_NAME).manifest.pot po/$(PACKAGE_NAME).metainfo.pot
 	msgcat --sort-output --output-file=$@ $^
 
+po/LINGUAS:
+	echo $(LINGUAS) | tr ' ' '\n' > $@
+
 # Update translations against current PO template
 update-po: po/$(PACKAGE_NAME).pot
 	for lang in $(LINGUAS); do \
@@ -68,12 +71,15 @@ watch:
 clean:
 	rm -rf dist/
 	rm -f $(SPEC)
+	rm -f po/LINGUAS
 
-install: $(WEBPACK_TEST)
+install: $(WEBPACK_TEST) po/LINGUAS
 	mkdir -p $(DESTDIR)/usr/share/cockpit/$(PACKAGE_NAME)
 	cp -r dist/* $(DESTDIR)/usr/share/cockpit/$(PACKAGE_NAME)
 	mkdir -p $(DESTDIR)/usr/share/metainfo/
-	cp org.cockpit-project.$(PACKAGE_NAME).metainfo.xml $(DESTDIR)/usr/share/metainfo/
+	msgfmt --xml -d po \
+		--template org.cockpit-project.$(PACKAGE_NAME).metainfo.xml \
+		-o $(DESTDIR)/usr/share/metainfo/org.cockpit-project.$(PACKAGE_NAME).metainfo.xml
 
 # this requires a built source tree and avoids having to install anything system-wide
 devel-install: $(WEBPACK_TEST)