Use node-gyp to infer GYP_MSVS_VERSION when possible.
Use node-gyp to infer GYP_MSVS_VERSION when possible.

As a refinement of my previous commit, we will continue setting the
$GYP_MSVS_VERSION variable to "2015" except in cases where the Python
library behind node-gyp succeeds in inferring the correct version.

Note that the Python library will never infer "2015" by itself, even
though that's the version recommended by the node-gyp setup instructions:

  https://github.com/nodejs/node-gyp#installation.

Note also that the Windows machines used by `meteor admin get-machine` and
build.meteor.com have version "2010e", which can be inferred.

In other words, this commit allows the following groups of developers to
avoid setting $GYP_MSVS_VERSION manually:

  1. Developers who followed the node-gyp setup instructions.
  2. Package authors who run `meteor publish-for-arch` on build machines.

mjmasn/meteor,chasertech/meteor,mjmasn/meteor,chasertech/meteor,Hansoft/meteor,Hansoft/meteor,chasertech/meteor,mjmasn/meteor,mjmasn/meteor,Hansoft/meteor,chasertech/meteor,Hansoft/meteor,Hansoft/meteor,chasertech/meteor,mjmasn/meteor,chasertech/meteor,chasertech/meteor,Hansoft/meteor,mjmasn/meteor,Hansoft/meteor,mjmasn/meteor