Auto-compile linenoise on build

[wilzbach]

Quite a simple idea to use the preBuildCommands hooks, but this should work on most Posix platforms as gcc is pretty universal. Of course, this isn't perfect and won't work on Windows.

• uses git subtree as DUB doesn't fetch git submodules on dub fetch
• Ideally DUB should provide such abstractions / build layers (e.g. interaction with deimos / native dependencies feels clunky dlang/dub#852, https://github.com/dlang/dub/wiki/DEP5)

[wilzbach]

Sorry for the commit noise, but there were some issues with the way I checked in the module with git subtree - it is resolved now & I also removed the linenoise installation from Travis to test that the following will work for any Posix system:

dub fetch drepl && dub run drepl

(and maybe just run soon).

[wilzbach]

CC @MartinNowak @ZombineDev - what's your opinion on this?

[MartinNowak]

With most other header libraries, we've integrated building with the deimos project, seems reasonable here as well.
E.g. https://github.com/s-ludwig/libgit2/tree/0.19
and https://github.com/biozic/d2sqlite3/blob/master/dub.sdl
That also solves any version incompats.

[MartinNowak]

Why do we want to copy that file here instead of using the deimos project as dependency?

[wilzbach]

OK moved to D-Programming-Deimos/linenoise#5
With that, only a simple git tag should be needed :)