diff -r 66e30c4870bb Lib/site.py
--- a/Lib/site.py	Thu Mar 21 15:02:16 2013 -0700
+++ b/Lib/site.py	Thu Mar 21 16:14:48 2013 -0700
@@ -120,7 +120,13 @@
     """Append ./build/lib.<platform> in case we're running in the build dir
     (especially for Guido :-)"""
     from sysconfig import get_platform
-    s = "build/lib.%s-%.3s" % (get_platform(), sys.version)
+    try:
+        s = "build/lib.%s-%.3s" % (get_platform(), sys.version)
+    except ImportError:
+        # see #17512, seen when trying to generate the _sysconfigdata.py
+        # on MacOS.  We don't need the builddir when generating this file
+        # during the core build
+        s = "build/lib.%s-%.3s" % (sys.platform, sys.version)
     if hasattr(sys, 'gettotalrefcount'):
         s += '-pydebug'
     s = os.path.join(os.path.dirname(sys.path.pop()), s)