Fix installation of shared library symlink (#535)

This fixes the regression from 29391ccd.

The dummy target that was removed in 29391ccd
was responsible for installing the unversioned symlink for the shared libraries.
The only part of this that was causing issue appears to be the dummy destination
that was only added as an workaround for old cmake version (pre 3.14-ish).
With the cmake version requirement bumped to 3.18 this should not be an issue anymore,
and we could also use the new NAMELINK_COMPONENT option to avoid using two install
statements.

This also fixes a bug from eb3d9778 causing
the libsleefgnuabi library to be missing the symlink...