This reverts commit c13f72ee0a6600963a10e57155d2e814d56599e2.

References #5447.

We use a Makefile which in turn uses cmake. If we wanted to set the install
prefix for cmake, we had to do this so far:

  make CMAKE_FLAGS="-DCMAKE_INSTALL_PREFIX=/tmp/nvim"

That's long and hard to remember. Following the conventions of other Makefiles,
this now works as well and is equivalent:

  make PREFIX=/tmp/nvim

Running tests from the top-level Makefile will use the third-party
dependencies from .deps instead of the ones from the Travis cache.

Specify that the ${GENERATED_HELP_TAGS} "command" (output) depends on
`helptags` so that it always regenerates the doc/ tags. (cmake "targets"
always run, whereas "commands" are contingent on their dependencies. But
we don't define doc/ dependencies because they are circular.)

Also add a functionaltest-lua target to run the functional tests using the lua
interpreter and corresponding helper to top-level Makefile

refs https://github.com/neovim/neovim/pull/2124#discussion_r26107174

Unlike Travis, `make test` currently only runs functional tests.
This can cause confusion since one might (understandably) think that `make
test` runs unit tests too, which it doesn't.

The `oldtest` target is still left out because it's quite slow and
Travis already runs it.

Source: https://github.com/mpeterv/luacheck
Docs:   http://luacheck.readthedocs.org/en/0.12.0/index.html

Run via "make testlint".