Support multiple Pythons in the same working tree

This way you can run the tests against several python versions by doing

  make PYTHON_VER=2.6 build test
  make PYTHON_VER=2.7 build test
  make PYTHON_VER=3.2 build test

repeatedly, without having to re-download and recompile the respective
clean Python interpreters.

Makefile

 HERE = $(shell pwd)
-PYTHON_PATH = $(HERE)/python
 PYTHON_VER ?= 2.7
 PYTHON_MINOR ?= 2.7.3
+PYTHON_PATH = $(HERE)/python$(PYTHON_VER)
 
 ifeq ($(PYTHON_VER),2.6)
 	PYTHON_MINOR = 2.6.8