cmake: fix rpath on shared libraries and executables

cmake: fix rpath on shared libraries and executables

The problem was due to the attempt of CMake to rewrite the RPATH of
the executables, or shared libraries, that didn't have any RPATH linked
during the building phase.

Currently INSTALL_RPATH is already set for all binaries. This patch
sets an empty INSTALL_RPATH for all binaries that don't depend on
any internal (also being built) target.

Signed-off-by: Ricardo Dias <rdias@suse.com>