Add is_child_theme(). Fixes #12998 props ptahdunbar.

git-svn-id: https://develop.svn.wordpress.org/trunk@14409 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/theme.php

@@ -6,6 +6,17 @@
  * @subpackage Template
  */
 
+/**
+ * Whether a child theme is in use.
+ *
+ * @since 3.0.0
+ *
+ * @return bool true if a child theme is in use, false otherwise.
+ **/
+function is_child_theme() {
+	return ( TEMPLATEPATH !== STYLESHEETPATH );
+}
+
 /**
  * Retrieve name of the current stylesheet.
  *