allow setting version and revision

[pendo324]

Issue #, if available:

Description of changes:
Allows setting the VERSION and REVISION variables for use in the Makefile, instead of always inferring them from the git repository. This is useful when building from the source tarball instead of a Git repository.

Testing performed:
Built a binary using VERSION=myversion REVISION=1234567 make soci soci-snapshotter-grpc and checked the output of --version commands:

./out/soci --version
soci version myversion 1234567

./out/soci-snapshotter-grpc --version
soci-snapshotter-grpc version myversion 1234567

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sondavidb]

I wonder if we want any sort of redundancy check. For example if we were to accidentally ship a binary with an incorrect version attached, it might be good to mention that this version was manually set with some indicator of sorts.

Not sure if it's a silly concern or not, though. It definitely seems like an edge case that's not likely to be a big problem, but would like your thoughts on that anyway.