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/flit-build-hook.sh

@@ -9,7 +9,7 @@ flitBuildPhase () {
     echo "Finished executing flitBuildPhase"
 }
 
-if [ -z "$dontUseFlitBuild" ] && [ -z "$buildPhase" ]; then
+if [ -z "${dontUseFlitBuild-}" ] && [ -z "$buildPhase" ]; then
     echo "Using flitBuildPhase"
     buildPhase=flitBuildPhase
 fi