Fixes:		d78291b0e973 ("lib/virtual_oss: don't (over)link to libsamplerate")
Reported by:	pkg-fallout
Sponsored by:	The FreeBSD Foundation
MFC after:	2 days

These plugins require samplerate.h due to virtual_oss's int.h including
it, but don't use any symbols directly so don't link to the library.

Centralize adding the include path.

Reviewed by:	christos
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D54705

general:
 - find libsamplerate's headers in the temporary sysroot instead of
   digging in the source tree.
 - use LIBADD where possible

lib/virtual_oss:
 - centralize SHLIBDIR define
 - centralize include path for internal headers
 - don't try to find libsamplerate directly in .OBJDIR

No functional changes.

Reviewed by:	christos
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D54704

This patch diverges quite a bit from the current upstream [1] in a few
ways:

1. virtual_oss(8), virtual_bt_speaker(8) and virtual_oss_cmd(8) are
   actually separate programs.
2. Backends (lib/virtual_oss) are built as separate shared libraries and
   we dlopen() them in virtual_oss(8) and virtual_bt_speaker(8) on
   demand.
3. virtual_equalizer(8) and the sndio and bluetooth backends are built
   as ports, because they depend on third-party libraries.
4. Use newer libav API in bluetooth backend (see HAVE_LIBAV ifdefs) to
   address compiler errors.

[1] https://github.com/freebsd/virtual_oss

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Reviewed by:	emaste
Differential Revision:	https://reviews.freebsd.org/D52308