fluxbox: explicitly configure optional dependencies

The following commit explicitly configures optional dependencies based
off the respective package selections for a given configuration.

In addition, this commit also cleans up some obsolete/incorrect
arguments. With the introduction of (at least) Fluxbox 1.3.7, the
'--with-imlib2-prefix' argument no longer exists. The argument
'--with-xpm-prefix' has never existed; it was accidentally committed
mimicking the configuration used to provide imlib2.

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>