[PATCH] Don't install embedded copy of GTest

[PATCH] Don't install embedded copy of GTest

Goole Test itself documents the `INSTALL_GTEST` option as something that projects embedding it should set to `OFF`. Leaving it on means that projects downstream from libraries that embed it are likely to encounter conflicting copies. This is particularly annoying because GTest does not maintain anything like a stable API, and so causes builds to fail if include paths pick up an inappropriately installed copy ahead of the one that the code wanted.

Fixes #488

Gbp-Pq: Name 1171.patch