Generate -fmacro-prefix-map to strip relative path from __FILE__.
Generate -fmacro-prefix-map to strip relative path from __FILE__.

This provides reproducible builds and minimizes the file path in debug messages, just like an in-tree make build.

For test source, prefix with test/ in case there are any module name collisions.

pgbackrest/pgbackrest,pgbackrest/pgbackrest,pgbackrest/pgbackrest