Better handling of unknown PYTHON_VER in Makefile

If PYTHON_VER is not known to the Makefile, don't fall back to
PYTHON_MINOR of 2.7.3, instead ask the user to specify a correct
PYTHON_MINOR explicitly.  Otherwise you get weird failures like #203.

Also allow the user to override PYTHON_MINOR on the `make` command line,
e.g.

    make PYTHON_VER=3.3 PYTHON_MINOR=3.3.6

Makefile

 HERE = $(shell pwd)
 PYTHON_VER ?= 2.7
-PYTHON_MINOR ?= 2.7.3
 PYTHON_PATH = $(HERE)/python$(PYTHON_VER)
 
 ifeq ($(PYTHON_VER),2.6)
-	PYTHON_MINOR = 2.6.8
+	PYTHON_MINOR ?= 2.6.8
+endif
+ifeq ($(PYTHON_VER),2.7)
+	PYTHON_MINOR ?= 2.7.3
 endif
 ifeq ($(PYTHON_VER),3.2)
-	PYTHON_MINOR = 3.2.3
+	PYTHON_MINOR ?= 3.2.3
 endif
-
 ifeq ($(PYTHON_VER),3.3)
-	PYTHON_MINOR = 3.3.0
-	PYTHON_ARCHIVE = Python-3.3.0
+	PYTHON_MINOR ?= 3.3.0
+endif
+
+ifndef PYTHON_MINOR
+    $(error Please specify desired PYTHON_MINOR for Python $(PYTHON_VER))
 endif
 
 PYTHON_ARCHIVE ?= Python-$(PYTHON_MINOR)