When building PPP 2.5.2 against older toolchains below 4, the build fails in sys-linux.c because <linux/ppp-ioctl.h> is not provided by those kernel headers. The header was introduced in later kernel versions, so toolchains with older headers cannot compile PPP without patching.

CC pppd-tls.o
sys-linux.c:133:29: fatal error: linux/ppp-ioctl.h: No such file or directory
#include <linux/ppp-ioctl.h>
^
compilation terminated.
Makefile:1334: recipe for target 'pppd-sys-linux.o' failed

My environment
Board: TI DM3730 (armv7l)
Kernel headers: 3.0
GCC: 4.9.4
libc: glibc 2.20