targets/(vdpau|xvmc): hardlink against the installed library

targets/(vdpau|xvmc): hardlink against the installed library

With commit 11e46a32aed and f9ebb1ea771 we resolved the symlink
generation required by the versioning of the library.
Although they incorrectly changed the way hardlinks are created by
linking to the ones from the build tree. If the device used for
building differs from the one set as destination linking will fail.

Reported-by: Andy Furniss <adf.lists@gmail.com>
Tested-by: Andy Furniss <adf.lists@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>