Makefile: export NO_SVN_TESTS

Without this patch, it really is not sufficient to define NO_SVN_TESTS
in config.mak or the Makefile.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/Makefile b/Makefile
index a0e90c0..cb83eba 100644
--- a/Makefile
+++ b/Makefile
@@ -656,6 +656,7 @@
 # with that.
 
 export NO_PYTHON
+export NO_SVN_TESTS
 
 test: all
 	$(MAKE) -C t/ all