Add option for multiple source directories and exclude filter

[davispuh]

This allows to specify multiple directories where to look for source code. Also exclude to filter certain files which doesn't need compilation.

[chuckremes]

Might be time to ping @headius or @enebo to see if they also got access to this repository after Wayne imploded. If not, then time to fork it and forget about it.

[davispuh]

I asked @headius about it on Twitter and he said he'll sort it and seems @enebo got it covered :)

[enebo]

Unfortunately, you will need to rebase from the last merge I did. It is no longer a clean merge.

Can I ask how common having multiple source directories is? It sort of makes sense this does happen in bigger projects but I am mostly just asking from a devil's advocate position (perhaps single source dir is just being opinionated?).

[davispuh]

I'm adding it because I would like to use ffi-compiler without restructuring nor changing upstream projects and compile them without making any changes to those files. Basically various projects have all kind of different Makefile configurations and goal is that ffi-compiler can be used as those makefile replacement. Currently you would either have to restructure layout or write your own ruby script to handle those differences and then compile, but this task could be done by ffi-compiler.

One example of such project would be libmspack for which I've implemented ruby-libmspack gem and that's mainly reason for these PR's along with few other projects.

[davispuh]

I just rebased this.

[enebo]

cool thanks. Your rationale makes total sense too :)