I\'m trying to build some third party libraries from source using Visual Studio 2015. I\'m creating the solution file using cmake 3.2.3 and I\'m using the ExternalProject_ad
It took a while and some research, but I think that I have reproduced your problem and found a solution. I found that cl.exe would seem to hang, but there was a process called "cvtres.exe" that was actually hanging. When I killed that process, the compilation would continue. There appears to be a known bug that KB3118401 seems to fix. I installed this patch and I haven't had the problem since. Hope this helps!
https://www.microsoft.com/en-us/download/details.aspx?id=51161