Use a tool dependency, not buildlink3, for PYTHON_FOR_BUILD_ONLY.

Index: lang/python/pyversion.mk
===================================================================
RCS file: /cvsroot/pkgsrc/lang/python/pyversion.mk,v
retrieving revision 1.105
diff -p -u -r1.105 pyversion.mk
--- lang/python/pyversion.mk	10 Dec 2012 03:15:48 -0000	1.105
+++ lang/python/pyversion.mk	23 Apr 2013 14:38:26 -0000
@@ -203,11 +203,13 @@ PYPKGPREFIX=
 PTHREAD_OPTS+=	require
 .include "../../mk/pthread.buildlink3.mk"
 
-.if defined(PYTHON_FOR_BUILD_ONLY)
-BUILDLINK_DEPMETHOD.python?=	build
-.endif
 .if defined(PYPKGSRCDIR)
-.include "${PYPKGSRCDIR}/buildlink3.mk"
+# XXX BUILD_DEPENDS/TOOL_DEPENDS split makes this variable name confusing.
+.  if defined(PYTHON_FOR_BUILD_ONLY)
+TOOL_DEPENDS+=	${PYDEPENDENCY}
+.  else
+.    include "${PYPKGSRCDIR}/buildlink3.mk"
+.  endif
 .endif
 
 PYTHONBIN=	${LOCALBASE}/bin/python${PYVERSSUFFIX}