[CMake] Disable parallel build under windows

[ycollette.nospam at free.fr]

Hello,

In a directory, I build some ExternalProjects.
Under windows, if I use the "NMake Makefiles" generator, the build is sequential (thanks to nmake which doesn't understand parallel build).
But if  I generate a Visual Studio solution, the external projects are build in parallel.
I would like to forbid the parallel build in my ExternalProjects directory. Is it possible to do via cmake ?

Best regards,

YC