Previously when ninja.bat was run from a prompt that already had VS vars
available, it was adding them a second time. Depending on what else the user
had in the path, this could cause the path to become too long and so things at
the end wouldn't be found. To avoid this, check if the compiler is available
first, and don't run the environment set up in that case.

R=maruel@chromium.org
Review URL: https://chromiumcodereview.appspot.com/10222010

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@133940 0039d316-1c4b-4281-b951-d872f2087c98