gdc phobos and druntime automatic import

[Benjamin Thaut]

I want to use a different version of druntime and phobos with GDC but I 
don't want to overwrite the default ones. Instead I want to specifiy a 
different include directory during compile time. I first thought that 
-nostdinc is the option I want, but -nostdinc seems to do something 
different. How does GDC know where to import druntime and phobos from, 
and how do I overwrite it?

Kind Regards
Benjamin Thaut