This is better practice in general, and allows to remove the "helptags
ALL" hacks.

Ref: https://github.com/neovim/neovim/issues/8824
Ref: https://github.com/neovim/neovim/commit/f1b67c3453c

* Makefile: fix dependencies with regard to helptags

- use the file as the main target to avoid unnecessary triggering
- use "make oldtest" on Travis to ensure it gets built

A separate rule it clearer, and allows for `make --keep-going lint` in
general later.

Ref: https://github.com/neovim/neovim/pull/10714

Ref: https://github.com/neovim/neovim/pull/10714#issuecomment-518932355

This avoids errors when using "make lint", but "flake8" is not
available.  We do not want to install it then via third-party.
On CI "make pylint" is used explicitly.

While not doing any harm with "Unix Makefiles", they do not work there
as-is.  Therefore just do not use them then.

Followup to #10366 (7f6ff829a).

This allows for `make build/bin/nvim`,
`make build/src/nvim/auto/funcs.generated.h` etc.

Ref: https://github.com/neovim/neovim/pull/10631#issuecomment-515756591

Append `CMAKE_INSTALL_PREFIX` to any given `CMAKE_EXTRA_FLAGS` always.

Regressed in 5031e3298.

Fixes https://github.com/neovim/neovim/issues/10524.

- Move .luacheckrc to root, add read_globals=vim
- Simplify lualint target, run it on all lua files
- Lint preload.lua, but ignore W211
- Remove testlint target, included in lualint (and lint)
- Clean up .luacheckrc

In order that `$(DEPS_BUILD_DIR)/Makefile` exists under parallel make