use the python we found in case it's not called "python"

/usr/bin/env python in vmodtool.py does not work for cases where it's called
python$VERSION, e.g. python2.7 - as is the case on SmartOS with pkgin

src/Makefile.am

@@ -21,7 +21,7 @@ vmod_example.3: vmod_example.man.rst
 vmod_example.lo vmod_example.man.rst: vcc
 
 vcc: @VMODTOOL@ $(top_srcdir)/src/vmod_example.vcc
-	@VMODTOOL@ $(top_srcdir)/src/vmod_example.vcc
+	@PYTHON@ @VMODTOOL@ $(top_srcdir)/src/vmod_example.vcc
 
 VMOD_TESTS = $(top_srcdir)/src/tests/*.vtc
 .PHONY: $(VMOD_TESTS) vcc