treewide: Make still dont* Variables are optional in most cases

Go beyond the obvious setup hooks now, with a bit of sed, with a skipped case:

 - cc-wrapper's `dontlink`, because it already is handled.

Also, in nix files escaping was manually added.

EMP

pkgs/development/node-packages/node-env.nix

@@ -363,7 +363,7 @@ let
 
         npm ${forceOfflineFlag} --nodedir=${nodeSources} ${npmFlags} ${stdenv.lib.optionalString production "--production"} rebuild
 
-        if [ "$dontNpmInstall" != "1" ]
+        if [ "''${dontNpmInstall-}" != "1" ]
         then
             # NPM tries to download packages even when they already exist if npm-shrinkwrap is used.
             rm -f npm-shrinkwrap.json