Install .debug files in system location if writable

If the installation prefix is set to /usr, then it is assumed that the
user has write access to the entire of that subtree, and that it would
be advantageous to put the .debug files in the /usr/lib/debug directory.
This is the usual case for package installations.

On the other hand, if the prefix is NOT /usr, then it is probably a
local installation; someone trying to install it to a more restricted
area. In that case, the default is to follow the binary, so that GDB
will still pick up the .debug file when loading the shared object.

In any case can this default be overridden, should it turn out that the
configuration script has guessed wrongly for your needs.