Support overriding path to VS without referring to the registry.

Add GYP_MSVS_OVERRIDE_PATH to point directly to a compiler installation.
Previously, the registry was always used to determine install paths for
compilers, but this allows pointing at a toolchain without requiring
any global state changes.
Review URL: https://codereview.chromium.org/11637025

git-svn-id: http://gyp.googlecode.com/svn/trunk@1556 78cadc50-ecff-11dd-a971-7dbc132099af