hexen: Add workaround for Mac Hexen IWAD.

The Mac version of hexen.wad is slightly different from the normal
DOS one: it contains a bunch of extra lumps but more importantly,
the GENMIDI and DMXGUS lumps are missing. This means that Chocolate
Hexen would crash on startup with the default settings (as OPL is
the default music output).

To work around this problem and allow the game to start up properly,
detect if the required lump is missing and adjust the music settings,
printing a helpful message to stdout to inform the user.
Merge branch 'master' of github.com:chocolate-doom/chocolate-doom