add custom CMake module path
add LibFindMacros CMake macro library
Makefile: add .deps/ to CMake prefix path

The CMake prefix path is the set of directories CMake searches for
libraries, header files, etc. Use the .deps directory we create when
building libuv as one of those locations.
automatically discover libuv in CMake

Idiomatically discover if libuv is installed.
Makefile: fetch deps before running CMake

CMake now required libuv so fetch it first.
make use of libuv when found by CMake
CMakeLists: remove hardcoded .deps directory

We use the standard CMAKE_PREFIX_PATH variable to pass the location of
.deps as a search location on the command line. There is now no need for
explicitly hard-coding it.
prefer linking to static libuv

Explicitly try to find the static libuv library first.

This might be considered a hack and if it weren't a single-use module it
might be preferable to control static versus shared preferences with a
configuration variable.
link to rt if it provides clock_gettime

As noted in #128, if clock_gettime is provided by librt then it does not
end up being linked into the static libuv.a binary. This might be
considered a bug in libuv but we can address it here.

Detect if librt provides the clock_gettime symbol and, if so, append it
to the list of libraries linked into nvim. On non-librt systems the
behaviour should be as before.