extensions, po-plug-ins: fix localization of extension metadata.

The metadata format being appdata, I simply use standard intltool
merging on the XML templates.
For Automake, I use a custom rule instead of INTLTOOL_XML_RULE because
the m4 macro hardcoded $(top_srcdir)/po as translation source directory
(yet we want to use po-plug-ins/ for these). Meson on the other side
already used a custom command.

extensions/goat-exercises/Makefile.am

@@ -128,7 +128,10 @@ appstream_files = $(appstream_in_files:.xml.in=.xml)
 appdatadir = $(goat_exercise_c_libexecdir)
 appdata_DATA = $(appstream_files)
 
-@INTLTOOL_XML_RULE@
+org.gimp.extension.goat-exercises.metainfo.xml: org.gimp.extension.goat-exercises.metainfo.xml.in $(wildcard $(top_srcdir)/po-plug-ins/*.po)
+	$(INTLTOOL_MERGE) $(top_srcdir)/po-plug-ins $< $(@) -x -u -c $(top_builddir)/po-plug-ins/.intltool-merge-cache
+
+.PHONY: org.gimp.extension.goat-exercises.metainfo.xml
 
 EXTRA_DIST = \
 	goat-exercise-gjs.js	\

extensions/goat-exercises/meson.build

@@ -110,7 +110,7 @@ appdatafile = custom_target(appdatafilename,
   output: [ appdatafilename, ],
   command: [
     intltool_merge,
-    po_dir,
+    po_plug_ins_dir,
     '@INPUT@',
     '@OUTPUT@',
     '--xml-style',

po-plug-ins/meson.build

@@ -1 +1,2 @@
+po_plug_ins_dir = meson.current_source_dir()
 i18n.gettext(gettext_package + '-std-plug-ins', preset: 'glib')