openbsd: Add minor number to shared library install name

This is what the OpenBSD porter's manual describes as correct, cf.
http://www.openbsd.org/faq/ports/specialtopics.html#SharedLibs

configure

@@ -2860,7 +2860,7 @@ case $target_os in
     openbsd|bitrig)
         disable symver
         SHFLAGS='-shared'
-        SLIBNAME_WITH_MAJOR='$(SLIBNAME).$(LIBVERSION)'
+        SLIB_INSTALL_NAME='$(SLIBNAME).$(LIBMAJOR).$(LIBMINOR)'
         SLIB_INSTALL_LINKS=
         oss_indev_extralibs="-lossaudio"
         oss_outdev_extralibs="-lossaudio"