In ce8be2385b5fee16859a630fca0c98ad290c8e21 ("cmake: set rpath for
libraries on linux") RPATH was first instroduced.
Later in 6d09efe8686de824adf3d8810bbe0d5c386643e4 ("Set RPATH only if
installation is done into non system directory") it was set only if it
was installed to non system directory.
But absolute RPATH not a good default, let's change this.
Fixes: #920