Implement the use-system-libuv build option that uvloop supports

This is a follow up to the discussion on: https://github.com/Vizonex/Winloop/issues/38

I was trying to package Winloop as a conda-forge package but I had issues with the statically linked libuv since conda-forge strongly discourages vendoring libraries (especially when that dependency already exists as a conda-forge package - which libuv does).

I inspected how uvloops conda-forge package was implemented and discovered uvloop has a use-system-libuv build option that lets it link dynamically against the DLL compiled libuv in the conda-forge package.

So this is an enhancement request to implement the same build option uvloop does to add extra flexibility for linking with libuv.

I left the conda-forge PR open to add winloop with the vendored libuv for now... but I'm not sure whether it'll be accepted or not. If dynamic linking could be optionally enabled it would resolve a lot of problems for deploying winloop there (and perhaps other places).

Start with the discussion in issue 38 and compare uvloop's setup.py build option and the linked conda-forge recipe. Inspect Winloop's build configuration to identify where an optional system libuv choice belongs; done means the option can dynamically link against the conda-forge libuv package while retaining the existing vendored build.