Require external gtest and remove it from submodules

[joeljonsson]

[cheesema]

Currently macOS requries the use of gtest external for some compilers. Lets make this change following the develop_joel update.

[cheesema]

Probably should do the same for blosc?

[cheesema]

Still need confirmation on how to install this directly on Linux through apt and have this all work.

[krzysg]

Just for sake of making it clear - what is a reason to require external gtest (or blosc)? Is there any problem with having it as submodule?

[cheesema]

It is a fair point, it may be un-necessary. But I have previously had various issues with compilation around gtest, that an external install seemed to solve.

Maybe one motivation is not suitable, its simply that blosc currently produces a bunch of warning, that during compilation produces a bit of noise.