gtkbuilder: Install gettext ITS files

Recent gettext has a feature to allow consumer projects to supply their
own string extraction rules for XML files, in ITS format.

Gettext still ships the rule for *.ui, but it would be better
maintained in the upstream project.

https://bugzilla.gnome.org/show_bug.cgi?id=760202

gtk/Makefile.am

@@ -1114,6 +1114,11 @@ pkgdata_DATA = gtkbuilder.rng
 
 EXTRA_DIST += gtkbuilder.rnc gtkbuilder.rng
 
+itsdir = $(datadir)/gettext/its
+its_DATA = gtkbuilder.loc gtkbuilder.its
+
+EXTRA_DIST += $(its_DATA)
+
 templates =				\
 	$(inspector_templates)		\
 	ui/gtkapplication-quartz.ui	\

gtk/gtkbuilder.its

@@ -0,0 +1,23 @@
+<?xml version="1.0"?>
+<its:rules xmlns:its="http://www.w3.org/2005/11/its"
+           xmlns:gt="https://www.gnu.org/s/gettext/ns/its/extensions/1.0"
+           version="2.0">
+  <its:translateRule selector="/interface" translate="no"/>
+  <its:translateRule selector="/interface//*[@translatable = 'yes']"
+                     translate="yes"/>
+
+  <!-- The 'comment' attribute should be extracted as a translator comment.  -->
+  <its:locNoteRule selector="/interface//*[@comments]"
+                   locNotePointer="@comments"
+                   locNoteType="alert"/>
+  <gt:escapeRule selector="/interface//@comments" escape="no"/>
+
+  <!-- The 'context' attribute should be extracted as msgctxt.  -->
+  <gt:contextRule selector="/interface//*[@context]" contextPointer="@context"/>
+
+  <its:preserveSpaceRule selector="/interface" space="preserve"/>
+
+  <!-- Extracted strings are consumed by the library and are never
+       merged back; we don't want to escape special characters.  -->
+  <gt:escapeRule selector="/interface" escape="no"/>
+</its:rules>

gtk/gtkbuilder.loc

@@ -0,0 +1,6 @@
+<?xml version="1.0"?>
+<locatingRules>
+  <locatingRule name="GtkBuilder" pattern="*.ui">
+    <documentRule localName="interface" target="gtkbuilder.its"/>
+  </locatingRule>
+</locatingRules>