[wplug] Clean way to check for glibc and version thereof?

[Hagbard Celine]

Hi peoples,

I'm screwing around with some network code, and have found that glibc still
does its own thing when it comes to structures for packets.  I happen to be
using glibc-2.3.2 on my Linux system, but would like this code to build under
linux-2.4.29/glibc-2.3.2 and FreeBSD-4.10 (for starters, more to come).

Can anyone point me to a good test for glibc's existence and version?
Preferably, it would run from autoconf (which would make it easy to add
-D_BSD_SOURCE/-D__FAVOR_BSD/-D_flavor_of_the_week to CFLAGS), but I suppose I
could live with it in the code...

Many much thanks,
HC