[VS Addin] Fix parallel building.

There was a bug in the calling code for compiler_wrapper.
New unit test added to prevent regression.

BUG=

Review URL: https://codereview.chromium.org/12248006

git-svn-id: https://nativeclient-sdk.googlecode.com/svn/trunk/src@1502 050acbb0-2703-11df-ab0a-9f3f633ae91d
7 files changed