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/auto-patchelf.sh

@@ -228,7 +228,7 @@ autoPatchelf() {
 # behaviour as fixupOutputHooks because the setup hook for patchelf is run in
 # fixupOutput and the postFixup hook runs later.
 postFixupHooks+=('
-    if [ -z "$dontAutoPatchelf" ]; then
+    if [ -z "${dontAutoPatchelf-}" ]; then
         autoPatchelf -- $(for output in $outputs; do
             [ -e "${!output}" ] || continue
             echo "${!output}"