scons: Silence clang 3.4 warnings on Ubuntu 12.04

This changeset fixes three types of warnings that occur in clang 3.4
on Ubuntu 12.04:

 * Certain versions of libstdc++ (primarily 4.8) use struct and class
   interchangeably. This triggers a warning in clang.

 * Swig has a tendency to generate code with the register class which
   was deprecated in C++11. This triggers a deprecation warning in
   clang.

 * Swig sometimes generates Python wrapper code which returns
   uninitialized values. It's unclear if this is actually a problem
   (the cases might be limited to failure paths). We'll silence these
   warnings for now since there is little we can do about the
   generated code.

SConstruct

diff --git a/SConstruct b/SConstruct
index d5f56b2..6250bdd 100755
--- a/SConstruct
+++ b/SConstruct
@@ -638,7 +638,12 @@
     # is relying on this
     main.Append(CCFLAGS=['-Wno-tautological-compare',
                          '-Wno-parentheses',
-                         '-Wno-self-assign'])
+                         '-Wno-self-assign',
+                         # Some versions of libstdc++ (4.8?) seem to
+                         # use struct hash and class hash
+                         # interchangeably.
+                         '-Wno-mismatched-tags',
+                         ])
 
     main.Append(TCMALLOC_CCFLAGS=['-fno-builtin'])