Implicit Declaration of function?

[Jason Price]

warning: implicit declaration of function ‘strptime’

I'm trying to do some basic Date parsing in Varnish.  But I get this
warning on compile.

Here's the relevant bits of VCL:

C{
#include <time.h>
#include <sys/time.h>
#include <stdio.h>
#include <stdlib.h>
}C

sub date_skew {
        C{
                time_t t = time(NULL);
                time_t digestepoch;
                struct tm tm;
                char *digestdate = VRT_GetHdr(sp, HDR_REQ, "\021x-digestdate:");
                if (!strptime(digestdate, "%a, %d %b %Y %H:%M:%S GMT", &tm)) {
                        VRT_SetHdr(sp, HDR_REQ, "\010dateparsefail:",
"1\0", vrt_magic_string_end);
                } else {
                        digestepoch = mktime(&tm);
                        if (abs(digestepoch - t) > 7200) {
                                VRT_SetHdr(sp, HDR_REQ,
"\010dateskewfail:", "1\0", vrt_magic_string_end);
                        }
                }
        }C
}

Note: Later on, I'll call error if either of the two headers above are set.

I've discovered that if I add:

#define _XOPEN_SOURCE 500

to the first C{ }C block, it passes muster.  Any clue why? (may not be
varnish related).

Varnish related: will that impact anything else inside varnish?

--Jason