Call vcvarsall.bat to set the build environment in the MSVC module

The current build environment setup for MSVC is quite broken, and will cause noticeable differences between QBS builds with Qt Creator (which calls vcvarsall.bat itself) and command line QBS builds.

To solve this problem permanently, QBS should obtain the proper environment variable settings by calling vcvarsall.bat with the correct target architecture argument in the MSVC module's setupBuildEnvironment property.

Documentation on vcvarsall.bat and the options it supports can be found here: http://msdn.microsoft.com/en-us/library/vstudio/x4d2c09s.aspx

Should QBS clear the environment it inherits from Qt Creator to prevent similar divergences in the future?