Oops, fixed a compile error from the previous commit
Overhauled project/build versioning.

* Existing CURRENT_PROJECT_VERSION setting is used as the "serial
  number" of the build. This will be set only by the automated build
  environment (not checked into the project.)
* A custom build setting CBL_VERSION_STRING defines the "marketing
  version number", e.g. "1.0" or "1.1 beta 3". This is checked into
  the project but will be overridden by the automated builds.
* An optional custom build setting CBL_SOURCE_REVISION contains the Git
  revision ID of the source code. This is only set by automated builds.
* The result of CBLVersionString() is generated from the project version
  and version string.
* The HTTP User-Agent header sent by the replicator is generated from
  the version string and OS/platform name (Mac OS X or iOS.)
* All three versions are logged at startup time using Log().