On UNIX, build only the parts of libuv we need for the filesystem,
process, and poll abstractions using the POSIX poll() backend. This
avoids many platform-specific conditions. On Windows, build all of
libuv; there are no conditional alternatives anyway.
Take logic from upstream `Makefile.am` and `configure.ac` to build libuv
sources.
Update `uv.h` to include KWSys Large File Support configuration so that
consistent stream libraries are used (on AIX with XL).
Add a `cm_uv.h` header to include the CMake-provided copy of the `uv.h`
header from CMake sources.
