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/build-support/setup-hooks/update-autotools-gnu-config-scripts.sh

@@ -1,7 +1,7 @@
 preConfigurePhases+=" updateAutotoolsGnuConfigScriptsPhase"
 
 updateAutotoolsGnuConfigScriptsPhase() {
-    if [ -n "$dontUpdateAutotoolsGnuConfigScripts" ]; then return; fi
+    if [ -n "${dontUpdateAutotoolsGnuConfigScripts-}" ]; then return; fi
 
     for script in config.sub config.guess; do
         for f in $(find . -type f -name "$script"); do