make: new target 'update-lang' to update the PO files for a languages against latest sources

With the new target, it is now easy to update all translations in the
project for a language against latest code with the simple command, run
from the top directory:

  make update-lang PO=<lang>

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

Makefile.am

@@ -40,6 +40,7 @@ EXTRA_DIST = TODO BUGS BUGFORM FAQ INSTALL \
 	email-clients.txt checkpatch.pl update-changelog.pl \
 	script/check-translation-sources.sh \
 	script/generate-mapfile-from-header.sh \
+	script/generate-po-from-template.sh \
 	script/generate-txt-from-texi.sh \
 	script/nested-func-to-macro.sh
 
@@ -56,3 +57,28 @@ coverage:
 
 .PHONY:	coverage-reset coverage
 endif
+
+# make update-lang PO=<lang>
+# ==========================
+# Update the PO files against the POT file in all the translation sub-directories
+#
+# We do not use an automatic recursive target from Automake (AM_EXTRA_RECURSIVE_TARGETS)
+# because we want to check only once that the variable PO was defined; the added bonus
+# being that we do not process all directories but only the related ones, which is
+# faster and a lot less verbose
+
+update-lang:
+if HAVE_XGETTEXT
+	@if echo "$(PO)" | grep -v '^[a-z][a-z]\(_[A-Z][A-Z]\)\?$$' > /dev/null ; then \
+	  echo "Error: invalid value \"$(PO)\" for update-lang, use PO=<lang>" >&2 ; exit 1 ; \
+	fi ; \
+	for subdir in $(SUBDIRS_PO); do \
+	  ($(am__cd) $$subdir && $(MAKE) $(AM_MAKEFLAGS) update-lang || exit $$?); \
+	done
+
+SUBDIRS_PO = WINGs/po po util/po WPrefs.app/po
+else
+	@echo "Error: the program 'xgettext' was not found by configure, it is mandatory for this operation" >&2 ; exit 1
+endif
+
+.PHONY: update-lang