virtual_oss: build system tidying

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

lib/virtual_oss:
- centra

virtual_oss: build system tidying

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

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