Including GDC in nixpkgs

[Stefan Frijters]

On Sunday, 22 September 2019 at 10:29:24 UTC, Stefan Frijters 
wrote:
> Hi Johannes,
>
> Thank you for the feedback. I've posted some more below so we 
> can try to find out what I'm doing wrong. I've added some blank 
> lines here and there to make things more readable, but it's all 
> very spammy I'm afraid.
>
> [...]

Well, I've made some progress:

* I still don't know what causes libtool to misbehave in 
parallel, but I've forced libphobos to be built with -j1, so 
that's at least a working workaround that doesn't slow down the 
rest of the build.
* The nixpkgs wrappers for gcc introduce the -idirafter flag, 
which is not accepted by gdc. It looks like this causes the check 
for the '-fPIC' flag to fail, and the flag to be dropped. This is 
the cause of the linker error for the shared libs. I've hacked 
around for a bit and got rid of the flag at the point of the 
check, so libphobos now builds correctly (but still with a bunch 
of warnings).
* I've added some nix code to generate a wrapper for gdc, like 
the ones that are generated for the rest of the GNU compiler 
family.

With these changes I can now build gdc + libraries and I've 
successfully built the example on the dlang.org front page from a 
nix shell.

The PR will need some reviewing from nixpkgs folks for sure since 
I did some nasty things.