build: Changing the way version is managed

Now control over the version number is moved to wscript.  In addition to
that, a new NDN_CXX_VERSION_BUILD_STRING macro is set to include more
detailed information, including commit ID (e.g., "0.1.0-rc1-1-g5c86570").

Change-Id: I89ddd4e2f1248bca4f828865ec6d3c3921fa2822