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/setuptools-check-hook.sh

@@ -12,7 +12,7 @@ setuptoolsCheckPhase() {
     echo "Finished executing setuptoolsCheckPhase"
 }
 
-if [ -z "$dontUseSetuptoolsCheck" ] && [ -z "$installCheckPhase" ]; then
+if [ -z "${dontUseSetuptoolsCheck-}" ] && [ -z "$installCheckPhase" ]; then
     echo "Using setuptoolsCheckPhase"
     preDistPhases+=" setuptoolsCheckPhase"
 fi