fluxbox: fix build against imlib2

* fix configure.in to take care of the given imlib2-prefix
* disable imlib2 support if imlib2 is not part of the selection

[Peter: imlib2 needs to be built with X support]
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>