darwin: add darwinMinVersionHook

Adding the hook allows the deployment target to be changed without
having to mess with the stdenv. The can also be propagated, which is
useful for libraries that have a minimum deployment target higher than
the default in nixpkgs. In that case, they can propagate the hook to
ensure library users are not targeting an unsupported version.

pkgs/os-specific/darwin/darwin-min-version-hook/setup-hook.sh

···
+local role_post
+getHostRole
+
+# Compare the requested deployment target to the existing one. The deployment target has to be a version number,
+# and this hook tries to do the right thing with deployment targets set outside of it, so it has to parse
+# the version numbers for the comparison manually.
+
+local darwinMinVersion=@deploymentTarget@
+local darwinMinVersionVar=@darwinMinVersionVariable@${role_post}
+
+local currentDeploymentTargetArr
+IFS=. read -a currentDeploymentTargetArr <<< "${!darwinMinVersionVar-0.0.0}"
+
+local darwinMinVersionArr
+IFS=. read -a darwinMinVersionArr <<< "$darwinMinVersion"
+
+local currentDeploymentTarget
+currentDeploymentTarget=$(printf "%02d%02d%02d" "${currentDeploymentTargetArr[0]-0}" "${currentDeploymentTargetArr[1]-0}" "${currentDeploymentTargetArr[2]-0}")
+
+darwinMinVersion=$(printf "%02d%02d%02d" "${darwinMinVersionArr[0]-0}" "${darwinMinVersionArr[1]-0}" "${darwinMinVersionArr[2]-0}")
+
+if [ "$darwinMinVersion" -gt "$currentDeploymentTarget" ]; then
+    export "$darwinMinVersionVar"=@deploymentTarget@
+fi
+
+unset -v role_post currentDeploymentTarget currentDeploymentTargetArr darwinMinVersion darwinMinVersionArr darwinMinVersionVar

pkgs/top-level/all-packages.nix

···
 
   plumed = callPackage ../development/libraries/science/chemistry/plumed { };
 
+  ### DEVELOPMENT / LIBRARIES / DARWIN SDKS
+
+  darwinMinVersionHook =
+    deploymentTarget:
+    makeSetupHook {
+      name = "darwin-deployment-target-hook-${deploymentTarget}";
+      substitutions = {
+        darwinMinVersionVariable = lib.escapeShellArg stdenv.hostPlatform.darwinMinVersionVariable;
+        deploymentTarget = lib.escapeShellArg deploymentTarget;
+      };
+    } ../os-specific/darwin/darwin-min-version-hook/setup-hook.sh;
+
   ### DEVELOPMENT / LIBRARIES / AGDA
 
   agdaPackages = callPackage ./agda-packages.nix {