Fix to Python version discovery.

CMakeLists.txt

@@ -212,12 +212,19 @@ ENDIF()
 
 # Add option to use Python 3.
 CMAKE_DEPENDENT_OPTION(NEKTAR_USE_PYTHON3
-    "If true, prefer to use Python 3." OFF "NEKTAR_USE_PYTHON3" OFF)
+    "If true, prefer to use Python 3." OFF "NEKTAR_BUILD_PYTHON" OFF)
 
 IF (NEKTAR_BUILD_PYTHON)
+    # Unset any existing python executable/library settings so that
+    # we can rediscover correct python version if v2/3 settings changed
+    unset(PYTHON_EXECUTABLE CACHE)
+    unset(PYTHON_INCLUDE_DIR CACHE)
+    unset(PYTHON_LIBRARY CACHE)
+    unset(PYTHON_LIBRARY_DEBUG CACHE)
+
     SET(PYTHONVER 2.7)
     IF (NEKTAR_USE_PYTHON3)
-        SET(PYTHONVER 3)
+        SET(PYTHONVER 3.0)
     ENDIF()
 
     # Find Python