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