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/interpreters/python/hooks/wheel-unpack-hook.sh

@@ -12,7 +12,7 @@ wheelUnpackPhase(){
     echo "Finished executing wheelUnpackPhase"
 }
 
-if [ -z "$dontUseWheelUnpack" ] && [ -z "$unpackPhase" ]; then
+if [ -z "${dontUseWheelUnpack-}" ] && [ -z "$unpackPhase" ]; then
     echo "Using wheelUnpackPhase"
     unpackPhase=wheelUnpackPhase
 fi