Modifying those functions to just return 0 helped. Apart from some warnings I now only get the following:

Code:
prime.o(.text+0xa713): In function `is_valid_double':
: undefined reference to `eisvaliddouble'
gw/mult.o(_TEXT32+0x5f): In function `_gwinfo1':
: undefined reference to `_CPU_L2_CACHE_SIZE'
collect2: ld returned 1 exit status
make: *** [sbfactor] Error 1
I don't understand the first one there is no is_valid_double function i prime.c.