Give libjim.so an soname

Reported-by: Markus Mayer <lotharlutz@gmx.de>
Signed-off-by: Steve Bennett <steveb@workware.net.au>
Fix hardcoded install paths

Many path inside the makefile are hardcoded, replace them with the
appropriate variables. (Main purpose: '$prefix/lib' is simply wrong
on 64bit systems).

Reported-by: Markus Mayer <lotharlutz@gmx.de>
Signed-off-by: Steve Bennett <steveb@workware.net.au>