[Nix-dev] Enabling Objective-C in Darwin stdenv

[shea at shealevy.com]

Hi all,

Building vim on OSX fails due to gcc lacking an Objective-C compiler.
Presumably, the vim build assumes that since the system is OSX we must be
using the xcode gcc, which does have Objective-C support. There are
probably other packages out there that make a similar assumption. The
obvious fix would be as follows:

1. Add a langObjC parameter (defaulting to false) to gcc's default.nix,
with behaviour analogous to the other lang* parameters.
2. Modify the darwin stdenv's gcc to set langObjC to true
3. (Optional, if anyone outside of darwin cares) Modify the default gcc to
set langObjC to true

Normally, these kinds of changes belong in stdenv-updates, but as this is
a potentially significant deficiency on darwin machines and we don't seem
to have too many darwin users, I'd like to perform steps 1 and 2 in trunk.
To be clear, this would NOT require a rebuild of stdenv on any non-darwin
system, but it would mean a total rebuild on any darwin systems. Would
anyone have a problem with this?

~Shea