scons: Silence clang 3.4 warnings on Ubuntu 12.04

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.