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

@@ -33,7 +33,7 @@ vmod_blobcode.lo: vcc_if.c vcc_if.h parse_encoding.h
 vcc_if.c: vcc_if.h
 
 vcc_if.h: @VMODTOOL@ $(top_srcdir)/src/vmod_blobcode.vcc
-	@VMODTOOL@ $(top_srcdir)/src/vmod_blobcode.vcc
+	@PYTHON@ @VMODTOOL@ $(top_srcdir)/src/vmod_blobcode.vcc
 
 VMOD_TESTS = $(top_srcdir)/src/tests/*.vtc
 .PHONY: $(VMOD_TESTS)