respect REBAR_BARE_COMPILER_OUTPUT_DIR env var during descriptor compilation

[the-mikedavis]

closes #204

this appears to fix my compilation with elixir 1.12

I'm a bit worried that this is just a band-aid fix of some other issue. I can't seem to hunt down what exactly changed in the elixir source that would cause this to be necessary

it looks like rebar recently added support for this environment variable (see erlang/rebar3#2237) which mix started passing to rebar in elixir 1.12

[the-mikedavis]

please correct me if I'm wrong here about the best way to format this little block: I have no idea what I'm doing with erlang formatting 😅

[tomas-abrahamsson]

It seems to be working fine, I tested just briefly also building with rebar3 and make and rebar2 too (I need to support that still). Nice! Could you squash the commits? (The case statement is fine, orelse in Erlang is short-circuiting (like || in C), whereas plain or is not, but I think both expect booleans as arguments, ie only the atoms true or false, so I think case is better in this case :) )

[the-mikedavis]

done and done!

I was fiddling around with an orelse but got some ArgumentErrors I didn't really understand, and it was working in my eshell. I really need to work on my erlang chops, huh? 😁

thanks again for maintaining gpb @tomas-abrahamsson 🙂

[tomas-abrahamsson]

Superduper! Looks good, and merged locally.

[tomas-abrahamsson]

Included in 4.17.1 which I just pushed. Thanks for the PR!