Install relative links to the soname.

Install relative links to the soname.

On make install, the library is installed to $(libdir)/$(realname)
(e.g. /usr/local/lib/libosl.so.0.1.2), and a link is created to
map $(linkername) to $(soname) (e.g. /usr/local/lib/libosl.so ->
/usr/local/lib/libosl.so.0).

This changes the ln command to create a relative link rather than an
absolute link. For example, /usr/local/lib/libosl.so now points to
libosl.so.0 without the leading directory. This shouldn't matter much
but it allows to move both files elsewhere without breaking the link.