use python2 tinstead of python to run waf

Author: Nicholas Knoblauch

src/Makevars

@@ -27,7 +27,7 @@ $(SHLIB): libbgen.a libzstd.a
 
 
 libbgen.a: $(BGEN_VERSION)
-	cd $(BGEN_VERSION) && ./waf configure && ./waf
+	cd $(BGEN_VERSION) && python2 ./waf configure && python2 ./waf
 	cp -f $(BGEN_VERSION)/build/libbgen.a .
 	cp -f $(BGEN_VERSION)/build/3rd_party/zstd-1.1.0/libzstd.a .
 	rm -rf $(BGEN_VERSION)/build