[CMake] VS2012 builds using v3.1.0 are a lot slower than v2.8.12

[Yves Frederix]

>> 3.1.0:    10m50s
>> 2.8.12:   4m02s
>> 3.1.0 (/MP): 6m29s
>> 2.8.12 (/MP): 3m48s
>
> I cannot reproduce that.  On my machine for OpenCV (as of commit
> 5e92a777) I get about the same time for either version of CMake.
>
>  3.1.0         : 8m57s
>  2.8.12.2      : 8m58s
>  3.1.0    (/MP): 4m52s
>  2.8.12.2 (/MP): 4m57s
>
> Check in your cases that 3.1.0 does not activate additional parts of
> the project that are not built under 2.8.12.
>
> Also please try a command-line build with just "msbuild" instead of
> going through "cmake --build":
>
>  msbuild ALL_BUILD.vcxproj /p:Configuration=Release /p:VisualStudioVersion=12.0

(shouldn't this have been VisualStudioVersion=11.0?)

So here are the timings:

3.1.0: 11m52s
2.8.12: 11m21s

When keeping an eye on the resource monitor, I can also see that,
while "cmake --build ." makes use of all the CPU cores (for 2.8.12) ,
this is not the case while running msbuild manually.

Yves