Summary:
This updates the top-level CMakeLists.txt file to compute package version
information, and expose this to C++ code in `eden-config.h`, and to Python
code in a new `eden/config.py` module.
Previously we exposed an `EDEN_VERSION` macro for the C++ code in
`eden-config.h`, but this was not initialized or used anywhere. Now the
top-level CMakeLists.txt file computes appropriate version information and
exposes the package name, version, release, commit ID, and build time in these
configuration files.
The version selection logic in CMakeLists.txt based largely on the code that
wez wrote for watchman in D20636833.
Reviewed By: wez
Differential Revision: D21000164
fbshipit-source-id: db1a1035f1eefec058bbad558d35e113005e454e
