configure script's dynamic linking check fails on openbsd

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=3941

Reported by Adam Montague, Sirius Hosting amontague@siriushosting.com

The LDSHARED setting for openbsd should look like:
OpenBSD*) LDSHARED="$REALCC -Wl,-Bshareable";;
instead of :
OpenBSD*) LDSHARED="ld -Bshareable";;

and LINKFORSHARED needs to look like:
OpenBSD*) LINKFORSHARED="-Wl,-E";;

Dynamic modules work fine after those changes are made.