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-catch-conflicts-hook.sh

@@ -5,6 +5,6 @@ pythonCatchConflictsPhase() {
     @pythonInterpreter@ @catchConflicts@
 }
 
-if [ -z "$dontUsePythonCatchConflicts" ]; then
+if [ -z "${dontUsePythonCatchConflicts-}" ]; then
     preDistPhases+=" pythonCatchConflictsPhase"
 fi