Use RUNPATH in installed library by default

When checking out from source code, the paths that were used to
configure the project should be carried on to the installed binary,
so the same libraries are used without much effort.

By using RUNPATH instead of (just) RPATH, it can be overridden with
LD_LIBRARY_PATH if the user so desires.
Exempt distribution packages from having RUNPATH

Files that are distributed as packages should always assume that all
the other libraries it depends on is as packages too, in the system
directories, so there is no need to have a RUNPATH embedded in them.
Merge pull request #296 from rolk/296_runpath

Embed original dependency paths in installed library