libthreadar: fix build on Darwin

pkgs/by-name/li/libthreadar/package.nix

···
     sha256 = "sha256-wJAkIUGK7Ud6n2p1275vNkSx/W7LlgKWXQaDevetPko=";
   };
 
+  postPatch = ''
+    # this field is not present on Darwin, ensure it is zero everywhere
+    substituteInPlace src/thread_signal.cpp \
+      --replace-fail 'sigac.sa_restorer = nullptr;' "" \
+      --replace-fail 'struct sigaction sigac;' 'struct sigaction sigac = {0};'
+  '';
+
   outputs = [
     "out"
     "dev"