Safety : Intermediate paths for Visual Studio

[elliotwoods]

Since now we have Win32 and x64 support....

If you switch between Win32 and x64, VS will build the intermediate files (.obj, .lib, etc) to the same locations. Which :

1. Can cause Visual Studio to get confused (resulting in needing to rebuild solution)
2. Each time you switch platform you overwrite the previous platform's files

I suggest that we change in openFrameworks.vcxproj

to

This should be transparent to the user.

cc: @DomAmato @liquidzym

[elliotwoods]

If we agree, I can PR this right away

[bakercp]

+1 on this.

[bakercp]

But also see this #3983.

[arturoc]

@elliotwoods yes if you can send a PR with this it would be great. we should do the same for xcode i think it has the same problem

[DomAmato]

Seems to work using the above method which is super convenient for testing the platforms +1