[Cmake] MS Project dsp regeneration

[Amitha Perera]

> It wipes them out if there are rules to build them like we have done in cmake
> generated dsp files.

How about having a build rule to build MyProject.dsp.cmake instead of
MyProject.dsp? CMake will generate this file, compare the contents
with an existing MyProject.dsp, and will overwrite MyProject.dsp if
necessary. This way, VS does not know that the dsps are autogenerated,
and won't wipe them. When the CMakeLists.txt changes, it'll be newer
than MyProject.dsp.cmake, so that will be regenerated, which will in
turn cause the corresponding .dsp to be updated, and only that project
need be reloaded.