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/patch-shebangs.sh

@@ -105,7 +105,7 @@ patchShebangs() {
 }
 
 patchShebangsAuto () {
-    if [ -z "$dontPatchShebangs" -a -e "$prefix" ]; then
+    if [ -z "${dontPatchShebangs-}" -a -e "$prefix" ]; then
 
         # Dev output will end up being run on the build platform. An
         # example case of this is sdl2-config. Otherwise, we can just