Better fallback for builds with no version.json and no git repo

In SConstruct:479 we try to extract version information using "git describe" when version.json is not present in the repo (which is the new default, see SERVER-21224).

Since there may be users building without a git repo it would be nice to add another fallback, such as environment variable or some other mechanism.

Workarounds include:

• adding a fake version.json file before compiling that looks as follows:

  {

  "version": "0.0.1-fake-"

  }

• Setting MONGO_VERSION in ~/.scons/site_scons/mongo_custom_variables.py:

  MONGO_VERSION="0.0.1-fake-"