Add soname to chibi shared library

Including a soname with a public shared library is a good practice and
is required by the policy of many distributions such as Debian.  The
major version of the soname distinguishes incompatible versions of the
shared library.
Use LDFLAGS when building chibi-scheme executable

This allows users or distributions to pass in additional LDFLAGS in
the usual place, such as the hardening flags applied by many
distributions.
Pass in LDFLAGS when building chibi libraries