GDB is missing on macOS

[edolstra]

Issue description

The GDB package (8.0.1) on macOS does not contain GDB:

$ nix build -f channel:nixos-17.09 gdb --argstr system x86_64-darwin

$ ls -l ./result/
total 16
dr-xr-xr-x 2 root root 4096 Jan  1  1970 include
dr-xr-xr-x 2 root root 4096 Jan  1  1970 lib
dr-xr-xr-x 2 root root 4096 Jan  1  1970 nix-support
dr-xr-xr-x 4 root root 4096 Jan  1  1970 share

The build log contains:

*** This configuration is not supported in the following subdirectories:
    zlib gdb sim

GDB 7.12.1 from the 17.03 channel did work.

[dtzWill]

Probably one of our configure flags isn't supported on Darwin? This should be easy to investigate/fix for someone with a Darwin machine.

[matthewbauer]

This was fixed in #36378