vcbuild - conditional regeneration

The attached patch changes vcbuild so the project and solution files are
only regenerated if they are actually changed. This helps when you're
developing in the Visual Studio GUI, because updating the files (even to
the same contents) will cause a reload and re-parse of all the projects
in the solution.

As a bonus, it doesn't trash the project file if it for some reason
aborts half-way through.

The patch includes part of the previously sent patch about pthreads
requirements, because they change the same area. Should be workable
without the other one as well, so it's no problem applying just one of
them if the other one is troublesome.

//Magnus