Provide a fallback if `ICMP6_ECHO_REQUEST' or `ICMP6_ECHO_REPLY' is not defined.

Provide a fallback if `ICMP6_ECHO_REQUEST' or `ICMP6_ECHO_REPLY' is not defined.

Hello,

I just started looking at liboping, looks great so far! I needed the attached
patch to compile on aix 5.2, works as expected once built. I chose to hardwire
the constant values due to conflicts when including aix's netinet/ip6_icmp.h.
I can try to untangle if desired but prefer to spend as little time on aix as
possible :)

% uname -a
AIX orange 2 5 000919934C00

% find /usr/include/ -name "*.h" | xargs grep ICMP6_ | egrep '128|129'
/usr/include/netinet/ip6_icmp.h:#define ICMP6_ECHO      128 /* echo service */
/usr/include/netinet/ip6_icmp.h:#define ICMP6_ECHOREPLY 129 /* echo reply */