point snd_soundfont to a default soundfont path on non-Windows systems

On non-Windows systems it is quite unlikely that the soundfont is
installed into the same path as the executable. On these systems,
point the snd_soundfont variable to the default soundfont path on
Debian, so that no further patching is required for this OS and its
derivatives.

prboom2/src/m_misc.c

@@ -374,10 +374,11 @@ default_t defaults[] =
    def_int,ss_none}, // number of audio events simultaneously // killough
 #ifdef _WIN32
   {"snd_midiplayer",{NULL, &snd_midiplayer},{0,"fluidsynth"},UL,UL,def_str,ss_none},
+  {"snd_soundfont",{NULL, &snd_soundfont},{0,"TimGM6mb.sf2"},UL,UL,def_str,ss_none}, // soundfont name for synths that support it
 #else
   {"snd_midiplayer",{NULL, &snd_midiplayer},{0,"sdl"},UL,UL,def_str,ss_none},
+  {"snd_soundfont",{NULL, &snd_soundfont},{0,"/usr/share/sounds/sf3/default-GM.sf3"},UL,UL,def_str,ss_none}, // soundfont name for synths that support it
 #endif
-  {"snd_soundfont",{NULL, &snd_soundfont},{0,"TimGM6mb.sf2"},UL,UL,def_str,ss_none}, // soundfont name for synths that support it
   {"snd_mididev",{NULL, &snd_mididev},{0,""},UL,UL,def_str,ss_none}, // midi device to use for portmidiplayer
 
 #ifdef _WIN32