Replace obsolete DATADIRNAME in configure.ac.in

Hi, we had a report that Parole didn't successfully build in Debian anymore: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1026648

After a bit of investigation, it seems that DATADIRNAME isn't really supported anymore (https://gitlab.xfce.org/apps/parole/-/blob/master/configure.ac.in#L143) and thus the pixmap installation dir ends up wrong (/usr/parole/pixmaps).

I've made a tentative patch at https://salsa.debian.org/xfce-team/apps/parole/-/blob/debian/master/debian/patches/0001-Replace-DATADIRNAME-by-datarootdir-in-configure.ac.patch but I'm honestly not totally sure it's suitable so I'd welcome your feedback before uploading that to Debian.