Add $RPM_CONFIGDIR/macros.d/ directory to default macro path (RhBug:846679)

Add $RPM_CONFIGDIR/macros.d/ directory to default macro path (RhBug:846679)

- There of course already is a drop-in directory for macro config bits
  in $(sysconfdir)/rpm/ but as /etc is supposed to be per-host admin
  territory, having another place for system package provided rpm config
  doesn't seem unreasonable.
- Whether this early in the path is the best possible or even reasonable
  position for this remains to be seen... but this way it could be used
  for "factory default" config bits by rpm itself as well, while still
  permitting vendor/host/user override.
(cherry picked from commit dcd261b9fe1e61e8ac3e99dbb443145bbf896886)