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/libraries/qt-5/hooks/qmake-hook.sh

@@ -31,6 +31,6 @@ qmakeConfigurePhase() {
     runHook postConfigure
 }
 
-if [ -z "$dontUseQmakeConfigure" -a -z "$configurePhase" ]; then
+if [ -z "${dontUseQmakeConfigure-}" -a -z "$configurePhase" ]; then
     configurePhase=qmakeConfigurePhase
 fi

pkgs/development/libraries/qt-5/hooks/wrap-qt-apps-hook.sh

@@ -64,10 +64,10 @@ qtOwnPathsHook() {
 
 preFixupPhases+=" qtOwnPathsHook"
 
-# Note: $qtWrapperArgs still gets defined even if $dontWrapQtApps is set.
+# Note: $qtWrapperArgs still gets defined even if ${dontWrapQtApps-} is set.
 wrapQtAppsHook() {
     # skip this hook when requested
-    [ -z "$dontWrapQtApps" ] || return 0
+    [ -z "${dontWrapQtApps-}" ] || return 0
 
     # guard against running multiple times (e.g. due to propagation)
     [ -z "$wrapQtAppsHookHasRun" ] || return 0