devel/sdl12: Add sndio backend

[tobik]

Summary

Add sndio backend

While here

- Add LICENSE_FILE, USES=localbase
- Strip library
- Group audio backend options
- Use options helpers for VGL

Approved by:	??? (mentor)
Obtained from:	OpenBSD
Differential Revision:	https://reviews.freebsd.org/D?????

Test Plan

poudriere testport 10.3/i386 ok (SNDIO=on and SNDIO=off)

[tobik]

devel/sdl12/files/patch-configure

39

I think this needs to be explained. OpenBSD regenerates the configure
script from configure.in after patching sndio detection into it.
However SDL 1.2 requires an older autoconf version (2.59) which is not
available on FreeBSD. Lots of errors when using autoconf 2.69.

Since I was unable to regenerate it I opted to edit configure
manually. So this does the bare minimum to wire the backend sources
into the build and make sure that SNDIO_CONFIGURE_ENABLE can do its
job in enabling/disabling sndio support.

[tobik]

Fix properties

[tobik]

and once more...

[lme]

Seems good. But please include the reasoning for the configure patch in the commit message.