I noticed the same thing about BOTHER not being defined. Like Jamey Sharp said, you can find it in <asm/termios.h>. Just a forewarning, I think I ran into problems including both it and the regular <termios.h> file at the same time.
Aside from that, I found with the glibc I have, it still didn’t work because glibc’s tcsetattr was doing the ioctl for the old-style version of struct termios which doesn’t pay attention to the speed setting. I was able to set a custom speed by manually doing an ioctl with the new style termios2 struct, which should also be available by including <asm/termios.h>:
struct termios2 tio;
ioctl(fd, TCGETS2, &tio);
tio.c_cflag &= ~CBAUD;
tio.c_cflag |= BOTHER;
tio.c_ispeed = 12345;
tio.c_ospeed = 12345;
ioctl(fd, TCSETS2, &tio);