detect Visual Studio build environment using vcvarsall.bat

To provide a consistent build environment for Visual Studio
setup-toolchains now executes the vsvarsall.bat that comes with the
Visual Studio installation. The environment is written to the profile
under the key "buildEnvironment".
The setupBuildEnvironment script in windows-msvc.qbs isn't needed
anymore.

Task-number: QBS-444
Task-number: QBS-508

Change-Id: I3191f5ff127bed6b96ce5ea5520b20fc9646364f
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>