pkgbase: set the prefix of debug files and macros to /usr/src

[bapt]

Summary

When the macro PACKAGE_BUILDING is set, then consider we are building package for pkgbase, this has already been used in Makefile.inc1 and reuse the PACKAGE_BUILDING macros already used for that purpose in the ports tree

[jrtc27]

This should be set in general for reproducible builds, to be honest, of which pkgbase is but one user.

[bapt]

I am fine with binding on the reproducible build variable

[emaste]

I agree with @jrtc27 that tying it to reproducible builds is sensible -- I'm fine with either change.
I'd like to understand the full workflow with gdb and lldb better.