add python3 checks to avoid confusion from errors with python2.

build_files/cmake/project_source_info.py

@@ -25,6 +25,14 @@ __all__ = (
     "SOURCE_DIR",
     )
 
+
+import sys
+if not sys.version.startswith("3"):
+    print("\nPython3.x needed, found %s.\nAborting!\n" %
+          sys.version.partition(" ")[0])
+    sys.exit(1)
+
+
 import os
 from os.path import join, dirname, normpath, abspath