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".

Fixes #3174

Problem:    With some regexp patterns the NFA engine uses many states and
            becomes very slow.  To the user it looks like Vim freezes.
Solution:   When the number of states reaches a limit fall back to the old
            engine. (Christian Brabandt)

https://github.com/vim/vim/releases/tag/v7-4-497

Helped-by: David Bürgin <676c7473@gmail.com>
Helped-by: Justin M. Keyes <justinkz@gmail.com>
Helped-by: Scott Prager <splinterofchaos@gmail.com>