Revert "don't need this anymore", need to fix cereal first

This reverts commit c1a62c6be9.

SConstruct

@@ -1,3 +1,5 @@
+import Cython
+import distutils
 import os
 import shutil
 import subprocess
@@ -17,6 +19,10 @@ AddOption('--asan',
           action='store_true',
           help='turn on ASAN')
 
+# Rebuild cython extensions if python, distutils, or cython change
+cython_dependencies = [Value(v) for v in (sys.version, distutils.__version__, Cython.__version__)]
+Export('cython_dependencies')
+
 real_arch = arch = subprocess.check_output(["uname", "-m"], encoding='utf8').rstrip()
 if platform.system() == "Darwin":
   arch = "Darwin"