Enable LTO

[springmeyer]

This enables Link Time Optimization for the vtquery build.

Learn more about why LTO is important at https://github.com/mapbox/cpp/blob/d7b1681d650f600940652744a31fd019195e9c31/glossary.md#link-time-optimization

This will ensure that vtquery code runs as fast as possible even if we split the code into more .cpp files as part of #14.

Next steps:

• I ported vtquery here to ensure this works, now that it does, it also needs to be backported to node-cpp-skel
• Run basic benchmarks on linux (inside docker container) to make sure code is actually just as fast (should see no difference).

refs mapbox/cpp#49

[springmeyer]

closing, this is landing in node-cpp-skel over in mapbox/node-cpp-skel#111. Will handle "forward-porting" separately. This PR was purely to test the concept.