Made the PYTHON3 cmake switch a proper cmake option.

dlib/add_python_module

@@ -11,11 +11,9 @@
 # python module to that folder when you run "make install".  Note that the path
 # given to install_*_to() is relative to your CMakeLists.txt file.
 #
-# Also note that if you set the cmake variable PYTHON3 equal to any non-zero
-# value then we will try and produce a Python 3 compatible library, otherwise
-# we will make a Pyhton 2 library.
 
 
+option(PYTHON3 "Build a Python3 compatible library rather than Python2." OFF)
 
 
 #SET(Boost_USE_STATIC_LIBS OFF)

tools/python/CMakeLists.txt

@@ -2,10 +2,6 @@
 CMAKE_MINIMUM_REQUIRED(VERSION 2.8.4)
 
 
-# Uncomment this to make a Python 3 compatible library instead of a Python 2
-# library.
-#set(PYTHON3 true)
-
 include(../../dlib/add_python_module)
 
 # Tell cmake to compile all these cpp files into a dlib python module.