Cannot make LDC use LTO when called via DUB

[Nordlöw via Digitalmars-d-learn]

I've added

buildType "release" {
         buildOptions "releaseMode" "optimize" "inline"
         dflags-posix-ldc "-flto=thin" "-Xcc=-fuse-ld=gold"
}
to my dub.sdl and built (on Linux) as

    /usr/bin/dub -v run --compiler=ldc2 --build=release

but neither the flag "-flto=thin" nor "-Xcc=-fuse-ld=gold" is 
propagated to the ldc2 call (at least no printed on stdout).

What's missing?