Fix BSD -rpath parameter

For BSD systems, Configure adds a shared_ldflags including a reference
to the Makefile variable LIBRPATH, but since it must be passed down to
Makefile.shared, care must be taken so the value of LIBRPATH doesn't
get expanded too early, or it ends up giving an empty string.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(cherry picked from commit c64879d3)