When CMake is invoked by the VS IDE re-run rule we compute whether or
not CMake really needs to re-run based on some timestamp helper files.
Previously we assumed that if the main generate.stamp file exists then
VS has correctly detected that the file is out of date.  However, this
assumption is too aggressive and re-runs CMake unnecessarily sometimes.

This commit removes the assumption and always checks timestamps itself.
The change breaks the explicit user re-run request (R-click -> Compile)
but only in cases when the build system is already up to date.