Generate an appropriate shebang line for MakeHeader script

Use configure.ac to handle platform differences where some
build hosts have only a python3, or only python, binary.

Related to https://github.com/htop-dev/htop/pull/6

scripts/MakeHeader.py.in

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env @PYTHON@
 import os, sys, string, io
 try:
    from StringIO import StringIO