CMake: Bump minimum version to 3.7.0 on Windows.
CMake: Bump minimum version to 3.7.0 on Windows.

The `SOURCE_SUBDIR` command to `ExternalProject_Add` was added in CMake
3.7.0, and is necessary to most cleanly build an external CMake built
project where the `CMakeLists.txt` is in a subfolder of the project.

Review: https://reviews.apache.org/r/58449/

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