Set the version number centrally in the build script.

This commit removes all uses of svn keyword expansion, and instead
arranges for WinURL's version number to be invented in Buildscr and
passed into the Windows binary build as a #define.


git-svn-id: svn://svn.tartarus.org/sgt@10270 cda61777-01e9-0310-a592-d414129be87e