Building systemd under Scientific Linux 6.4

Unfortunately this is not possible, as, at the end of a long chain
of prerequisites (compiling autoconf, automake, m4, utillinux (blkid),
dbus the last requisite cannot be met:
udev-builtin-input_id.c:166: error: 'BTN_TRIGGER_HAPPY' undeclared.

This 'BTN_TRIGGER_HAPPY' should be provided by kernel headers, but seems
to be intruduced in the headers of kernel 2.6.34 (linux/input.h).

Scientific Linux uses 2.6.32.