The command that works is
dmd a.d b.owhere b.o is a precompiled c file, similar to https://github.com/dlang/druntime/blob/master/src/core/stdc/errno.c
When using rdmd it doesn't work anymore. When I make rdmd --chatty, I can find the reason: b.o is ommited in the call of dmd. How can I make rdmd pass this parameter to dmd too?