Including GDC in nixpkgs

[Stefan Frijters]

As a fan of both D and the nixpkgs package manager I wanted to 
try and help include GDC in nixpkgs, but I have been running into 
some problems.

If anyone could help by taking a look at my pull request at 
https://github.com/NixOS/nixpkgs/pull/69144 that would be greatly 
appreciated.

Current issues include:

* When compiling without forcing --disable-shared, there are 
linker errors about needing to compile with -fPIC. It seems that 
some files are built using that flag, but not all, and I'm not 
sure how to go about forcing it for the rest.
* When invoking nix-build without specifying --cores 1 there are 
problems when building libphobos; it looks like object files are 
moved before they are generated. This problem goes away when I 
build on a single core, but of course everything takes a very 
long time to try out.
* I have disabled bootstrapping for now to speed up the debugging 
process, hopefully it can be enabled again later on.

I have taken the compiler flags to be used from 
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=gdc-git, 
which was linked in another thread on this forum, but maybe this 
is too specific for Arch?

Thanks in advance.