Failed to build for Linux 2.6.38 or earlier

[Low-power]

When trying to build uptimed for Debian 6, I encountered this error:

urec.c: In function 'read_uptime':
urec.c:140: error: 'CLOCK_BOOTTIME' undeclared (first use in this function)
urec.c:140: error: (Each undeclared identifier is reported only once
urec.c:140: error: for each function it appears in.)

According to man page clock_gettime(2), from a more recent version of Debian GNU/Linux, CLOCK_BOOTTIME was added since Linux 2.6.39; while the Linux UAPI provided by Debian 6 is 2.6.32.

Considering Linux 2.6.32 isn't that ancient, I would like to see uptimed to support it out-of-box.

Btw, the target platform for this build is a 50 MHz i486-based computer; the hardware information is available at https://linux-hardware.org/?probe=3764b249f4 in case anyone interested.