Fix Mac build, patch #1091 by Humberto Diogenes.
diff --git a/Lib/plat-mac/buildtools.py b/Lib/plat-mac/buildtools.py
index 2422612..1545014 100644
--- a/Lib/plat-mac/buildtools.py
+++ b/Lib/plat-mac/buildtools.py
@@ -17,7 +17,8 @@
 warnings.warn("the buildtools module is deprecated", DeprecationWarning, 2)
 
 
-BuildError = "BuildError"
+class BuildError(Exception):
+    pass
 
 # .pyc file (and 'PYC ' resource magic number)
 MAGIC = imp.get_magic()