Make libuv handling more idiomatic for CMake

The build for #3 is failing at the moment but this addresses a similar issue. Instead of a) not checking for libuv and b) hard-coding the ``.deps`` directory, use a more idiomatic way of handling libuv. As noted in the description for #99 it is more usual for one to write a custom CMake module to find any compile-time dependencies. This PR adds such a mechanism for libuv and also uses the standard ``CMAKE_PREFIX_PATH`` variable to tell CMake where to find the non-system libraries we install.

This allows compilation with the system libuv (if any) when using the standard CMake ``mkdir build && cd build && cmake .. && make`` workflow. The practical upshot of this being that packaging neovim for, e.g., Debian using a system-supplied libuv is trivial: neovim will behave like a completely standard CMake-based bit of software.

Since CMake now checks for the presence of libuv, move the fetching and compiling of it to the ``make cmake`` target.