Strange behaviour of rdmd vs. dmd concerning main function

[berni via Digitalmars-d-learn]

Something similar happend now, but this time it works with dmd 
and rdmd produces the error:

The command that works is

dmd a.d b.o

where 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?