Fix the x64 Windows build environment used by the buildbots.  %VS90COMNTOOLS%\vsvars32.bat is fine for 32-bit builds, but doesn't work for x64 builds, regardless of /MACHINE:AMD64 and /USECL:MS_OPTERON flags passed to cl.exe.  Launch the x86_64 cross compilation environment via '%VS90COMNTOOLS%\..\..\VC\vcvarsall.bat x86_amd64'.  I don't have access to any systems *without* Visual Studio 2008 Professional installed (i.e. just Express Edition), so I can't test if x64 compilation works w/ VS Express at the moment.

Additionally, force a clean in our build.bat files before building, just whilst we're going through these build system updates.

And finally, add in the missing MACHINE=AMD64 option to our Tcl/Tk x64 build.