build: do not assume glibc on linux

The whole IPv6 stack detection could need refactoring. But this
fixes the linux check to not assume glibc. Fixes build against
musl c-library.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
1 file changed