wmaker: add script to check the call-back function used when loading configuration

wmaker: add script to check the call-back function used when loading configuration

Because the C compiler cannot check the consistency between the type of the
variable being pointed to, and the type expected by the call-back function
that is used to parse and store the value from the configuration file,
there is a risk of mismatch that can cause Window Maker to misbehave due to
data corruption, which depends strongly on the configuration values and the
architecture on which Window Maker is running.

This patch introduces a script that checks the consistency where possible, to
raise the issues when performing "make check".

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>