treewide: Make more dont* variables OK to be undefined in setup hooks

pkgs/development/libraries/qt-5/hooks/qtbase-setup-hook.sh

@@ -61,6 +61,6 @@ postPatchMkspecs() {
         fixQtBuiltinPaths "$dev/mkspecs" '*.pr?'
     fi
 }
-if [ -z "$dontPatchMkspecs" ]; then
+if [ -z "${dontPatchMkspecs-}" ]; then
     postPhases="${postPhases}${postPhases:+ }postPatchMkspecs"
 fi