Use -rpath (or -rpath-link) when linking

Use -rpath (or -rpath-link) when linking

It avoids the need to use LD_LIBRARY_PATH when running programs linked
against custom libraries.  The default build types have been amended so
that debug builds use an absolute rpath.  This allows them to be run from
the source tree.  Release builds use a relative rpath to make them more
easily relocatable in the filesystem.