Drop “gtk+” plus sign in GtkBuilder <requires>.

demos/gtk-demo/blendmodes.ui

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface>
-  <requires lib="gtk+" version="3.20"/>
+  <requires lib="gtk" version="3.20"/>
   <object class="GtkWindow" id="window">
     <property name="resizable">0</property>
     <property name="title">CSS Blend Modes</property>

demos/gtk-demo/filtermodel.ui

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface>
-  <requires lib="gtk+" version="3.16"/>
+  <requires lib="gtk" version="3.16"/>
   <object class="GtkListStore" id="liststore1">
     <columns>
       <column type="gint"/>

demos/gtk-demo/font-features.ui

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface>
-  <requires lib="gtk+" version="3.12"/>
+  <requires lib="gtk" version="3.12"/>
   <object class="GtkWindow" id="window">
     <property name="default-width">600</property>
     <property name="default-height">500</property>

gtk/gtkbuilder.c

@@ -71,7 +71,7 @@
  * element contains an `<object>` element which describes the child object.
  * The target toolkit version(s) are described by <requires> elements,
  * the “lib” attribute specifies the widget library in question (currently
- * the only supported value is “gtk+”) and the “version” attribute specifies
+ * the only supported value is “gtk”) and the “version” attribute specifies
  * the target version in the form “`<major>`.`<minor>`”. The builder will error
  * out if the version requirements are not met.
  *

gtk/gtkbuilderparser.c

@@ -2014,7 +2014,7 @@ end_element (GtkBuildableParseContext  *context,
        * required versions, possibly throw a signal allowing them
        * to check their library versions here.
        */
-      if (!strcmp (req_info->library, "gtk+"))
+      if (!strcmp (req_info->library, "gtk"))
         {
           if (!GTK_CHECK_VERSION (req_info->major, req_info->minor, 0))
             {

gtk/ui/gtkplacesview.ui

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface domain="gtk40">
-  <requires lib="gtk+" version="3.16"/>
+  <requires lib="gtk" version="3.16"/>
   <object class="GtkListStore" id="completion_store">
     <columns>
       <column type="gchararray"/>

gtk/ui/gtkplacesviewrow.ui

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface domain="gtk40">
-  <requires lib="gtk+" version="3.16"/>
+  <requires lib="gtk" version="3.16"/>
   <template class="GtkPlacesViewRow" parent="GtkListBoxRow">
     <property name="width-request">100</property>
     <property name="child">