fix(CMakeLists.txt): increase thread stack space on OSX
fix(CMakeLists.txt): increase thread stack space on OSX

closes #1721

On OSX, the default thread stack space is quite small (512Kb), and
each stack frame is quite big in Debug mode.

johoelzl/lean,digama0/lean,Kha/lean,digama0/lean,leodemoura/lean,fpvandoorn/lean,leanprover-community/lean,leanprover-community/lean,leanprover-community/lean,leanprover-community/lean,dselsam/lean,digama0/lean,fpvandoorn/lean,leanprover-community/lean,leanprover/lean,fpvandoorn/lean,leanprover/lean,fpvandoorn/lean,Kha/lean,leodemoura/lean,leodemoura/lean,avigad/lean,Kha/lean,Kha/lean,dselsam/lean,leodemoura/lean,fpvandoorn/lean,fpvandoorn/lean,dselsam/lean,johoelzl/lean,leanprover/lean,leanprover/lean,leanprover-community/lean,dselsam/lean,Kha/lean,leanprover/lean,avigad/lean,leanprover/lean,leodemoura/lean,avigad/lean,johoelzl/lean,leodemoura/lean,johoelzl/lean,dselsam/lean,digama0/lean,digama0/lean,avigad/lean,johoelzl/lean,digama0/lean,johoelzl/lean,dselsam/lean,avigad/lean,avigad/lean,Kha/lean