Automatically build tools required to compile

[0xb01u]

Fixes #131.

Heavily inspired by pokeemerald's build process.

I am no Makefile expert by any means, and whatever I used to know is a bit rusty nowadays. So there may be cleaner ways of achieving the same result. But I think this is good enough, and makes the compilation slightly more convenient. I am open to suggestions on how to improve the PR, though.

Also adds support for in-project agbcc installation (using its install.sh script), instead of requiring to add agbcc to the $PATH. Either of those options works now. (I find it more convenient.)

[biosp4rk]

Typo

[0xb01u]

Oh, also, one advantage this solution has is that it is easily extensible to other tools. We currently only have gbafix and preproc as pre-build requirements. But, for example, if we added tools for data, image or sound processing, and they had a Makefile, we would just have to add them to make_tools.mk for them to be built automatically.

[0xb01u]

Ah, btw, this does not automatically extract the data from the ROM. That part was not automatized, and is required to be done before compiling (e.g., make extract && make).

Let me know if you would like to have that automatized too, and I'll update the PR.