Build libuv using the same CMake machinery as other dependencies

[damonbarry]

These changes are incomplete--they work on macOS but I haven't added support yet for Linux or Windows (unfortunately the build instructions for libuv are a little different on each platform). But a related conversation came up in #280, so I'll post this now for people to look at.

[damonbarry]

I've updated the build for Windows and Linux too. I've confirmed that all platforms build successfully with their build scripts (build.sh or build.cmd), and that proxy_sample works.

[damonbarry]

I pushed a few more commits:

• I turned native remote module support OFF by default in CMake, to be consistent with other features. That means --disable-native-remote-modules is no longer valid in the build.(sh|cmd) build scripts. Instead, there's a new flag: --enable-native-remote-modules.
• If you already have libuv installed (from an installer or package manager, or because you built it from source and did make install to the system default location, e.g. /usr/local), then we'll use that instead of trying to build/install it ourselves. This is consistent with how we handle our other dependencies.

NOTE on macOS I had to upgrade XCode developer command line tools in order to get past a weird CMake error. I did xcode-select --install from the terminal, and it launched a UI which upgraded the tools to being them in line with the latest XCode (which was recently updated on my Mac).

[darobs]

Drop the --enable-dotnet-core-binding here?

[damonbarry]

This sample (dotnet_core_dynamic_add_module) is specifically about the .NET Core binding, so I think it's correct as-is.