Recent updates to gtest break the build of `parallel-hashmap`

[barracuda156]

@greg7mdp Could we have an option to use an external (“system”) gtest? Current master does not build, at least without some hacks, so while parallel-hashmap 2.0.0 built fine earlier when I updated it, now MacPorts ran into the problem: macports/macports-ports#29657

I have reported the issue to gtest upstream too: google/googletest#4860

MacPorts’ gtest (its current version or updated to the latest release) should work fine.

[greg7mdp]

Fixed by #288, thank you @barracuda156 .

[barracuda156]

Thanks for merging! That was fast.