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/python-imports-check-hook.sh

@@ -10,7 +10,7 @@ pythonImportsCheckPhase () {
     fi
 }
 
-if [ -z "$dontUsePythonImportsCheck" ]; then
+if [ -z "${dontUsePythonImportsCheck-}" ]; then
     echo "Using pythonImportsCheckPhase"
     preDistPhases+=" pythonImportsCheckPhase"
 fi