Add package specific variables to External_*.cmake

[Erotemic]

Applies the pattern used in #339 to define variables relevant to a package in their External_*.cmake. This PR just defines the variables (so I can use them in my caffe2 branch), but does not simplify the corresponding places in the Caffe and Caffe_Segnet build scripts. This does cause temporary duplication of code, but it also makes it easier to review.