Support non-default locations of MSVC in Scons

Our new Windows 10 bots come with a prepackaged MSVC, which does not
live in the default location that Scons expects.
Make msvc_exists always return true, and allow use of MSVC_USE_SCRIPT to
skip other env setup that will be done in NaCl's site_tools instead.

Bug: 1336569
Change-Id: Ibe25ddf207404f349155c498f43dd36bf13b29bc
Reviewed-on: https://chromium-review.googlesource.com/c/native_client/src/third_party/+/3773653
Reviewed-by: Derek Schuff <dschuff@chromium.org>
1 file changed