llvm also has a copy of gtest in its source tree. This causes cmake to bork
since it sees several different targets with the same name (gtest and
gtest_main). So we have to rename our versions of gtest and gtest_main to
something else... We're just appending _ycm now.
This will cause pain when we want to update gtest in the future from upstream,
but I don't see a better way of handling this.
