Windows: Fixed CMake generator for Protobuf target.
Windows: Fixed CMake generator for Protobuf target.

This fixes the build when using MSVC 2017 instead of MSVC 2015.
The Protobuf target now uses the same CMake generator as the
project, instead of hardcoding MSVC 2015.

Review: https://reviews.apache.org/r/58446/

jpeach/mesos,andschwa/mesos,asamerh4/mesos,Gilbert88/mesos,dforsyth/mesos,reneploetz/mesos,gsantovena/mesos,andschwa/mesos,chhsia0/mesos,zmalik/mesos,reneploetz/mesos,chhsia0/mesos,abudnik/mesos,asamerh4/mesos,verizonlabs/mesos,jpeach/mesos,neilconway/mesos,dforsyth/mesos,kaysoky/mesos,neilconway/mesos,dforsyth/mesos,gsantovena/mesos,jpeach/mesos,verizonlabs/mesos,asamerh4/mesos,jpeach/mesos,Gilbert88/mesos,zmalik/mesos,gsantovena/mesos,neilconway/mesos,verizonlabs/mesos,dforsyth/mesos,Gilbert88/mesos,Gilbert88/mesos,verizonlabs/mesos,zmalik/mesos,abudnik/mesos,verizonlabs/mesos,kaysoky/mesos,andschwa/mesos,zmalik/mesos,dforsyth/mesos,abudnik/mesos,chhsia0/mesos,verizonlabs/mesos,andschwa/mesos,abudnik/mesos,andschwa/mesos,reneploetz/mesos,reneploetz/mesos,reneploetz/mesos,abudnik/mesos,andschwa/mesos,zmalik/mesos,Gilbert88/mesos,neilconway/mesos,kaysoky/mesos,abudnik/mesos,asamerh4/mesos,chhsia0/mesos,gsantovena/mesos,dforsyth/mesos,andschwa/mesos,kaysoky/mesos,jpeach/mesos,chhsia0/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,asamerh4/mesos,verizonlabs/mesos,reneploetz/mesos,Gilbert88/mesos,zmalik/mesos,kaysoky/mesos,jpeach/mesos,chhsia0/mesos,abudnik/mesos,zmalik/mesos,gsantovena/mesos,neilconway/mesos,gsantovena/mesos,asamerh4/mesos,jpeach/mesos,neilconway/mesos,asamerh4/mesos,neilconway/mesos,gsantovena/mesos,chhsia0/mesos,dforsyth/mesos,kaysoky/mesos