gtest undermines pure-c libs use in static builds on windows

[cyrush]

Still narrowing in on the exact issue, but I think this is something we should resolve in BLT's copy of gtest.

Here are more details: https://github.com/LLNL/conduit/pull/238

And a simpler example I am working on: https://github.com/cyrush/2017_12_cmake_obj_libs_issue

[cyrush]

I think gtest is the culprit, here is the simple example failing: https://ci.appveyor.com/project/cyrush/2017-12-cmake-obj-libs-issue

Also, given the flags that gtest is setting, this probably undermines all pure-c static libs, not just object libs