While introspection is pulled in already. The USE_GNOME=introspection
sets a enviroment variable to prevent introspection from generating the
/root/.cache directory.

Index: graphics/entangle/Makefile
===================================================================
--- graphics/entangle/Makefile	(revision 339601)
+++ graphics/entangle/Makefile	(working copy)
@@ -19,7 +19,7 @@
 
 USES=		gmake pkgconfig
 GNU_CONFIGURE=	yes
-USE_GNOME=	gtk30
+USE_GNOME=	gtk30 introspection:build
 
 # XXX: it would be nice if one day this option would be passed automatically
 # to configure scripts that support it :)