DMD release compiler flags when building with GDC

[Per Nordlöw]

I've noticed that the make flag ENABLE_LTO=1 fails as

     Error: unrecognized switch '-flto=full'

when building dmd with GDC 9.

Does gdc-9 support lto? If so what flags should I use?

If not what are the preferred DFLAGS when building dmd with gdc?