make: generate English .pot file via xgettext

Needs the optional gettext-tools package to run.

Makefile

@@ -52,6 +52,10 @@ health:
 	# check test script output and advertise a failure...
 	[ "`${.CURDIR}/src/etc/rc.php_test_run`" == "FCGI-PASSED PASSED" ]
 
+translate:
+	find src | xargs xgettext -L PHP --from-code=UTF-8 -F \
+	    -o src/share/locale/en/LC_MESSAGES/OPNsense.pot
+
 clean:
 	git reset --hard HEAD && git clean -xdqf .