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/interpreters/python/hooks/flit-build-hook.sh

@@ -9,7 +9,7 @@ flitBuildPhase () {
     echo "Finished executing flitBuildPhase"
 }
 
-if [ -z "${dontUseFlitBuild-}" ] && [ -z "$buildPhase" ]; then
+if [ -z "${dontUseFlitBuild-}" ] && [ -z "${buildPhase-}" ]; then
     echo "Using flitBuildPhase"
     buildPhase=flitBuildPhase
 fi