libsigsegv: fix build on x86_64 (probably others too)

The configure script doesn't detect that the target is posix-compliant
and tries to use a custom version of segfault analysis that actually
doesn't build. Most likely, it's because the configure script doesn't
support linux-3.x. Anyway, we can just tell configure that we're
posix-compliant.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>