[CMake] Parallel builds

[Greg Marr]

On Wed, Sep 16, 2015 at 5:55 PM, Cory Quammen <cory.quammen at kitware.com> wrote:
> VTK exposes the /MP compiler flag in its CMake configuration.
>
> Here is the relevant code from
> http://www.vtk.org/gitweb?p=VTK.git;a=blob;f=CMake/vtkDetermineCompilerFlags.cmake;h=1398050afb34ff8c0a74137d847c19a6f63b12e9;hb=HEAD
>
> 126 # Enable /MP flag for Visual Studio 2008 and greator
> 132     SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /MP${CMAKE_CXX_MP_NUM_PROCESSORS}")
> 133     SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /MP${CMAKE_CXX_MP_NUM_PROCESSORS}")

If you're just going to set it to the number of processors on the machine, you might as well just use /MP, which means use the number of processors on the machine.  The only time you need a number is when you want to use a different number of parallel processes, such as only using half your capacity because something else is using the other half, or leaving one or two cores open for GUI work.