Groundwork for a more versatile interface for using
sound libraries. A lot of sound libraries work across
multiple platforms.
The current NetHack sound stuff is quite limited.
Binaries can have a variety of window ports linked into
them, and it makes sense to have something similar for
sound.
This tries to set things up in a more soundlib-centric way,
rather than inserting things in a platform-centric way.
It establishes a new top-level directory sound (akin to win
for the window interface routines, or "window-port") where
sound-related additions and sndprocs and support files can be
added and used across platforms.
The default interface is nosound and the 'nosound' interface
is in src/sounds.c
The interface for 'windsound', which contains the same minimal
USER_SOUNDS support using built-in routines that has been in the
windows port for a long time is added to
sound/windsound/windsound.c.
For now, the sound interface support for 'qtsound' has been added
to the existing Qt files win/Qt/qt_bind.h and win/Qt/qt_bind.cpp,
and a note has been placed in sound/qtsound/README.md to avoid
confusion.
New header file added: include/sndprocs.h.
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