Support for googletest 1.10.0

[tavolodobela]

I'd like you to consider adding support for googletest 1.10.0.

Currently ion-c checks out version 1.7.0 of googletest. The problem is that we have other projects depending on a more recent version of googletest, so we face a version conflict: project A depends on ion-c which ships googletest 1.7.0, while project B depends on googltest 1.10.0..

[almann]

I did recently fix #183, which should eliminate gtest and friends from the install target--does this address your issue?

[tavolodobela]

I'll check, and keep you posted.