Make sure it doesn't try to build for darwin if leading-underscore is off.

Author: angerman

rts/rts.cabal

@@ -287,7 +287,7 @@ flag static-libzstd
   manual: True
 flag leading-underscore
   default: False
-  manual: False
+  manual: True
 flag unregisterised
   default: False
   manual: True
@@ -352,8 +352,8 @@ library
       DerivedConstants.h
       stg/MachRegsForHost.h
 
-    if !flag(leading-underscore)
-      if os(darwin)
+    if os(darwin)
+      if !flag(leading-underscore)
         buildable: False
 
     if arch(javascript)