cmake: add USE_SYSTEM_* options

[ConnorBaker]

Similar to the work done in pytorch/pytorch#37137, this adds the following CMake options:

• USE_SYSTEM_LIBS
• USE_SYSTEM_GOOGLEBENCHMARK
• USE_SYSTEM_GOOGLETEST

This is particularly useful in the context of Nix, where we can build these libraries once and then re-use them elsewhere to avoid rebuilding vendors dependencies.

[ConnorBaker]

@malfet @Maratyszcza would either of you have the bandwidth to take a look at this?

[malfet]

@ConnorBaker Looks good to me, though I wonder if USE_SYSTEM_${LIBNANE} option is at all needed. I.e. what would be the realistic scenario, when one wants to use gtest from OS, but gmock from third_party folder.

[ConnorBaker]

Thank you for the feedback! I've addressed comments and force-pushed the changes.

@malfet I think it makes sense to have USE_SYSTEM_LIBS to avoid needing to set multiple flags. With respect to the question about gmock: I thought gmock was brought in by find_package(GTest)? Or was this a question of how to support system gtest but third-party gmock?

@Maratyszcza are the names good now? Also (because I have multiple PRs open doing the same thing), would you recommend using that naming instead of GOOGLE_*?

[Maratyszcza]

@ConnorBaker I don't have a preference for GOOGLETEST vs GOOGLE_TEST, so long as the naming is consistent