Fixed logic error in detecting 'Alt' key.

DirtyHairy, it looks like you removed a negation and replaced it with
a positive (ie, you removed an _ifndef_, not an _ifdef_) :)

configure

@@ -843,4 +843,4 @@ if test "$_host_os" = darwin; then
 WARNING: plain UNIX-style builds on OSX without XCode have degraded functionality
 and are unsupported. Continue on your own risk...
 EOI
-fi
+fi

src/emucore/EventHandler.hxx

@@ -168,9 +168,9 @@ class EventHandler
     inline bool kbdAlt(int mod) const
     {
   #if defined(BSPF_MAC_OSX) || defined(OSX_KEYS)
-      return (mod & KBDM_ALT);
-  #else
       return (mod & KBDM_GUI);
+  #else
+      return (mod & KBDM_ALT);
   #endif
     }