treewide: *Phase(s)? variables are optional

If these aren't defined, the stdenv defaults are used in the `*Phase`
case, or no extra phases are done, in the `*Phases` case.

pkgs/development/tools/build-managers/gn/setup-hook.sh

@@ -9,6 +9,6 @@ gnConfigurePhase() {
     runHook postConfigure
 }
 
-if [ -z "${dontUseGnConfigure-}" -a -z "$configurePhase" ]; then
+if [ -z "${dontUseGnConfigure-}" -a -z "${configurePhase-}" ]; then
     configurePhase=gnConfigurePhase
 fi