cmake: Fix setup hook on bash < 4.0

This matters in nix-shell and stdenvNative on Darwin.

Fixes #3204.
Hack to fix llvm/clang build on Darwin

(Or at least restore it to the same level as brokenness as before the
staging merge. I.e. it depends on gcc-apple, which doesn't build on
standard 10.9 systems.)
Merge branch 'staging'