fix build (gtest 1.6 -> 1.7)

fix build (gtest 1.6 -> 1.7)

Summary:
D1494614 migrated us to gtest 1.7 which broke multiple builds (with
"error: 'tuple' is already declared in this scope" coming from
https://code.google.com/p/googletest/source/detail?spec=svn621&r=621).
This diff makes folly compatible with both versions of gtest, and the rest
of fbcode is now compatible with gtest 1.7.

Test Plan: fbconfig -r folly unicorn/diskindex4 && fbmake opt -j32

Reviewed By: jiayan@fb.com

Subscribers: fbcode-common-diffs@, chaoyc, search-fbcode-diffs@, sdoroshenko, cold-storage-diffs@, unicorn-diffs@

FB internal diff: D1497389

Tasks: 4909974, 4910547, 4910567, 4909983