[sdlsound] SDL_sound modplug configure patch

[Ryan C. Gordon]

> Repost, since this got missed.
> 
> SDL_sound-1.0.1 checks for modplug.h just under /usr/include/modplug.h, when 
> it should be checking /usr/include/libmodplug/modplug.h, hence it never finds 
> and uses modplug.  Patching configure.in and adding 
> -I/usr/include/libmodplug/ to the include path lets it detect and use it.  
> I'm not sure what the proper way to add include paths is, besides the 
> commandline, however.

Something like this is in Subversion now (both 1.0 and 1.1 branches),
thanks!

--ryan.