Support for VC2012 and its included Windows SDK 7.1A.
Specifically:
* Windows SDK scons tool: hard code paths for Windows SDK 7.1A as it comes with no batch files.
* Windows SDK scons tool: don't worry about setting up MSVC tool at the end as we'll list this in the tools before the windowsSDK, plus it looks like scons 2.3 fixed a few issues we originally had.
* sconstruct: set up 32 bit and 64 bit environments (env32 and env64) with msvc/sdk tools in sconstruct rather than in nvdaHelper/sconscript. Then set env as env32. this allows signtool to be found with sdk 7.1A.
