[RFC] build: fix running of functional tests directly with CMake

If you aren't just building everything into build/, then the functional
tests fail because they can't find the nvim executable.  Let's pass in
the location of the nvim executable, and set NVIM_PRG environment
variable accordingly.
It's called a generator expression, and in this case `TARGET_FILE` retrieves the path to the specified target (`nvim`).  If you look at the [documentation for `add_custom_command`](http://www.cmake.org/cmake/help/v2.8.12/cmake.html#command:add_custom_command) you can see there are several available.  These are also available for `add_custom_target`.