fix datadir on OpenBSD

Index: src/supertux/main.cpp
--- src/supertux/main.cpp.orig
+++ src/supertux/main.cpp
@@ -212,10 +212,7 @@ void PhysfsSubsystem::find_datadir() const
     }
     else
     {
-      // if the game is not run from the source directory, try to find
-      // the global install location
-      datadir = basepath.substr(0, basepath.rfind(INSTALL_SUBDIR_BIN));
-      datadir = FileSystem::join(datadir, INSTALL_SUBDIR_SHARE);
+      datadir = "${PREFIX}/share/supertux2";
     }
   }
 
