Upgrade bundled gtest and make sure it does not clobber system gtest

Merged Povilas Kanapickas requested to merge p12tic/xorg-gtest:gtest-upgrade into master

On my Ubuntu system I can't use the bundled gtest because it's installed at the same path as system gtest. This PR moves the headers to $prefix/include/xorg/gtest-lib (couldn't think of a better path).

Additionally, the gtest itself is upgraded to fix a couple of bugs.

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