libuv: Add support for building on IBM i (OS400)

IBM i ("OS400") builds were failing due to improper compilation of libuv when cmake is configured with --no-system-libuv. Added logic based on libuv's Makefile.am located at https://github.com/libuv/libuv/blob/v1.x/Makefile.am

FWIW, it looks like libuv's CMakeLists.txt needs some work for AIX and IBM i, which I will pursue separately.

Currently WIP until I complete testing, but am open to comments as I have low confidence I took the correct approach.

Backport: release