DMD release compiler flags when building with GDC

[Per Nordlöw]

On Sunday, 10 November 2019 at 15:59:22 UTC, Johannes Pfau wrote:
> I think -flto is the proper flag for GCC/GDC. I don't know if 
> LTO is working though. A long time ago there were some bugs, 
> but maybe that's been fixed. You probably just have to try and 
> see ;-)

Thanks