Hi,
I come back because I've finally found a solution to my problem :
I added this line to my postinst script :
/usr/lib/fw4spl > /etc/ld.so.conf.d/fw4spl.conf && ldconfig
and this line to my postrm script :
rm /etc/ld.so.conf.d/fw4spl.conf && ldconfig
Hoping it will help someone someday... ;-)
Corentin