Use new GST_ENABLE_EXTRA_CHECKS #define

https://bugzilla.gnome.org/show_bug.cgi?id=756870
diff --git a/configure.ac b/configure.ac
index 85080dc..8546023 100644
--- a/configure.ac
+++ b/configure.ac
@@ -78,6 +78,7 @@
 dnl *** check for arguments to configure ***
 
 AG_GST_ARG_DISABLE_FATAL_WARNINGS
+AG_GST_ARG_ENABLE_EXTRA_CHECKS
 
 AG_GST_ARG_DEBUG
 AG_GST_ARG_PROFILING