error: Unexpected use of HDL library function(s)

Upon further testing, this seems to be related to the datatype. If I use "unsigned long" in the RTL library, I get the error. If I use "unsigned long long" it works. So there seems to be a mismatch of the inferred width between the different compiler parts. (I guess the vague C standard is partly to blame here, but a better error message would have saved me a lot of time).