fix gtest depency fallback (#152)
fix gtest depency fallback (#152)

Summary:
fixes issue where ExternalProject_Add in CMakeLists is unreachable
fixes #153
Pull Request resolved: https://github.com/facebook/wangle/pull/152

Reviewed By: yfeldblum

Differential Revision: D15053099

Pulled By: Orvid

fbshipit-source-id: 56c00689e6124246c548063700195c8d2da3485b

facebook/wangle,facebook/wangle,facebook/wangle