Remove cmake policies no longer in use:
    CMP0026 can go away, we no longer use the LOCATION property.

Simplify INSTALL_DEBUG_TARGET.
Now that we know the location of executables, plugins and convenience
libraries we don't need the LOCATION property.

Calls to INSTALL_DEBUG_TARGET were spread out over misc. cmake files.
Collect them in one place for static libraries.

This is a manual/partial backport of

commit f16f089d3c17c22a44274383976218f9b6a58d08
Author: Tor Didriksen <tor.didriksen@oracle.com>
Date:   Wed Sep 13 10:42:00 2017 +0200