Instead of aborting on an empty buffer we just continue, since the
buffer would be freed anyway on the next line. This patch makes the
function more tolerant.

The original install rule would fail if the link already existed.
Now installing succeeds if the destination was already there, it will
be overwritten with the new file.

The extern keyword is needed for instructing the linker to resolve
hw_image_hooks_glx, which is declared in another object. Failing to
do so results in an assert later on when using subpictures.