OS X builds failing with "C compiler cannot create executables"

[Glenn Morris]

Hi,

I see some OS X builds have been failing for some weeks in the configure
step with

  configure: error: C compiler cannot create executables
  See `config.log' for more details

Eg this fails: http://hydra.nixos.org/build/30904978

But eg this one works: http://hydra.nixos.org/build/30853497

Examining config.log:

  configure:5019: checking for gcc
  configure:5046: result: clang
  [...]
  configure:5315: checking whether the C compiler works
  configure:5337: clang -O1 conftest.c >&5
  xcode-select: error: no developer tools were found at
  '/Applications/Xcode.app', and no install could be requested (perhaps
  no UI is present), please install manually from 'developer.apple.com'.


That seems like a general problem that should be fixed?
Independent of that, is there a way to specify gcc in a release.nix file?

Thanks.