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/ruby-modules/gem/default.nix

@@ -99,7 +99,7 @@ stdenv.mkDerivation ((builtins.removeAttrs attrs ["source"]) // {
     runHook preUnpack
 
     if [[ -f $src && $src == *.gem ]]; then
-      if [[ -z "$dontBuild" ]]; then
+      if [[ -z "''${dontBuild-}" ]]; then
         # we won't know the name of the directory that RubyGems creates,
         # so we'll just use a glob to find it and move it over.
         gempkg="$src"