xcbuild: move setup hook to xcbuildHook

Not every package that needs xcbuild will want to use its build phase.
I have moved the xcbuild setup hook to the new attribute xcbuildHook.
This means that dontUseXcbuild is no longer needed. If you just need
to call xcbuild on its own you can just refer to xcbuild.

pkgs/development/tools/xcbuild/setup-hook.sh

@@ -19,11 +19,9 @@ xcbuildInstallPhase () {
     runHook postInstall
 }
 
-if [ -z "$dontUseXcbuild" ]; then
-    buildPhase=xcbuildBuildPhase
-    if [ -z "$installPhase" ]; then
-        installPhase=xcbuildInstallPhase
-    fi
+buildPhase=xcbuildBuildPhase
+if [ -z "$installPhase" ]; then
+    installPhase=xcbuildInstallPhase
 fi
 
 # if [ -d "*.xcodeproj" ]; then