build: better support for python3 systems

Improve support for systems where `python` is actually `python3`.

Not all systems have a `python2` binary, so simply updating the shebang
won't work.

What we can do is apply some cleverness: start life as a shell script,
locate the python binary, then re-execute the script but this time as
python code.

Special care is taken to ensure that spaces in arguments are passed on
verbatim.

configure

@@ -1,4 +1,15 @@
-#!/usr/bin/env python
+#!/bin/sh
+
+# Locate python2 interpreter and re-execute the script.  Note that the
+# mix of single and double quotes is intentional, as is the fact that
+# the ] goes on a new line.
+_=[ 'exec' '/bin/sh' '-c' '''
+which python2.7 >/dev/null && exec python2.7 "$0" "$@"
+which python2 >/dev/null && exec python2 "$0" "$@"
+exec python "$0" "$@"
+''' "$0" "$@"
+]
+del _
 
 import sys
 if sys.version_info[0] != 2 or sys.version_info[1] not in (6, 7):