Refactor followup: enable LTO about carmen-cache HOT 7 CLOSED

@apendleton I'll get a working example going in node-cpp-skel next

Here is a working example you can follow: mapbox/vtquery#63

Up to now I've used binutils-gold for linking and not lld and I'd like to give lld a shot.

Ideally need to do this mapbox/mason#561 for easy lld usage, so I think sticking with binutils linker (per the vtquery example PR above) is ideal.

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@apendleton yes, LTO requires a consistent major clang++ version to be used. That is my oversight: so due to the external dep we'll need to:

• move to doing mason build rocksdb rather than mason install rocksdb to trigger source compiles
• force the same compiler, which will require a slight tweak to mason to allow us to override the default (old) apple compilers at https://github.com/mapbox/mason/blob/6aa9591a336ac71bfa1974185b6ee784a9b6033a/mason.sh#L71-L72.

So, I recommend you continue the cleanup work without this LTO addition, to make sure you'r not blocked. We can hit it as a followup once the mason fix is available.

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@springmeyer it looks from cursory Googling like lld already supports LTO, so just to be sure I'm understanding: this should just be adding -flto to cppflags and ldflags in our binding.gyp, right?

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@apendleton I'll get a working example going in node-cpp-skel next and then ping you when ready. Up to now I've used binutils-gold for linking and not lld and I'd like to give lld a shot.

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@springmeyer we gave this a go and it seems not to be working, I think because we're trying to link to previously-built static libraries from Mason that were compiled with a different LLVM version (rocksdb, etc.). We get this error:

/home/travis/build/mapbox/carmen-cache/mason_packages/.link/bin/ld: error: LLVM gold plugin has failed to create LTO module: Invalid value (Producer: 'LLVM5.0.0git-30c4647' Reader: 'LLVM 3.9.1')

see for reference the job: https://travis-ci.org/mapbox/carmen-cache/jobs/346996737

Thoughts on how to proceed?

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Mason fix is looking good at #119

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Done in #116

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