version: report commit hash as "build number" in long version string
version: generate "build number" from commit timestamp

- cmake: git_timestamp() returns last commit time formatted as
  `YYYYMMddHHmm`.
- Always include commit hash in :version and --version output.

`nvim --version` sample output:
  NVIM 0.0.0-alpha+201410070245 (compiled Oct  7 2014 05:30:45)
  Commit: f747b2b1ff7bfe7eb00cc2be82d7af87c98f1111
version: remove "Compilation" and "Linking" messages.

Can't reliably get this information from cmake (#1267), so it's
misleading to show these messages at all. We can always revert this
commit if we find a way later.