PF_CAN is defined by

#define PF_CAN		AF_CAN

in linux/socket.h :-(

Making the change in configure.in didn't lead to any change: no error when I ran configure (which is ./configure --with-py-debug in my case), and when I build, the same error (AF_CAN undefined) occurs. Just to eliminate any extraneous variables and be absolutely sure, the program

#include <sys/socket.h>
#include <stdio.h>

int main(int argc, char ** argv)
{
    printf("AF_CAN is %d\n", AF_CAN);
}

also fails with the same error.