vitual_oss: build system tidying

[brooks]

Summary

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.

Sponsored by: DARPA, AFRL

[christos]

Neither lib/virtual_oss nor usr.sbin/virtual_oss build with this patch:

root@freebsd:/mnt/src/lib/virtual_oss # make
Building /usr/obj/mnt/src/amd64.amd64/lib/virtual_oss/null/null.pico
In file included from /mnt/src/lib/virtual_oss/null/null.c:39:
/mnt/src/usr.sbin/virtual_oss/virtual_oss/int.h:33:10: fatal error: 'samplerate.h' file not found
   33 | #include <samplerate.h>

[brooks]

In D54704#1249776, @christos wrote:

Neither lib/virtual_oss nor usr.sbin/virtual_oss build with this patch:

root@freebsd:/mnt/src/lib/virtual_oss # make
Building /usr/obj/mnt/src/amd64.amd64/lib/virtual_oss/null/null.pico
In file included from /mnt/src/lib/virtual_oss/null/null.c:39:
/mnt/src/usr.sbin/virtual_oss/virtual_oss/int.h:33:10: fatal error: 'samplerate.h' file not found
   33 | #include <samplerate.h>

Direct manual building when you depend on a library that isn't installed isn't supported in FreeBSD's build system. You must build in a make buildenv environment having previously bootstrapped includes (it also won't link since libsamplerate presumably isn't installed.)