Lroundf: undefined reference

i want to compile asterisk on my soho-router. in general it is working as long as i don’t use fax-support.
but i aim to use asterisk for faxes as well.
i compiled everything i need beforehand (spandsp, libtiff etc) successfully. in general it looks good, because i can check “res_fax” and “res_fax_spandsp” in the configuration-menu.

But during compiling i get the error message.
it occurs in file.c in line 845, where the function roundf is called.

when i comment this line compiling succeeds, but the causes asterisk to crash (i guess because this missing function, maybe a division by 0?).

now i am searching for help! where can get this needed lroundf-function?