Update gtest version to 1.8.1

[jslee02]

This also fixes the build failure on Ubuntu Bionic.

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[brianhou]

Were there any changes to code that we've written, or is the only change to update gtest code? If it's the latter, what's the rationale for not just using a git submodule?

[jslee02]

I believe there is one thing we changed to suppress build warnings by adding SYSTEM option to include_directories(...) (see this diff). Let me make the change for this PR as well.

Alternatively, git submodule can be used with custom build script instead of using the provided one by gtest, if git submodule is preferred.

[brianhou]

Hmm, this actually makes me mildly concerned. What if we update gtest again in the future, and don't remember that that change (or other future changes we make) were necessary?

New proposal: what if we fork gtest into PRL, and apply our changes. We can then merge in new versions when desired, while maintaining our own changes. I'm somewhat fuzzy on exactly where libherb and other libraries that use aikido find gtest, but maybe that could be useful for those as well?

[jslee02]

Hmm, this actually makes me mildly concerned. What if we update gtest again in the future, and don't remember that that change (or other future changes we make) were necessary?

Agreed.

The proposal sounds reasonable to me.

As a note, libherb download gtest (using git) when it gets built instead of having the code inside the repo. I personally don't prefer this way because PPA doesn't allow to download code while building the package.

Anyways, I don't have any strong preference for this. Any method that is convenient to maintain would be great.

[brianhou]

Let's merge for now so we can unbreak Travis. @jslee02 can you make an issue to figure out how to deal with this? Thanks!