Using g_get_user_data_dir() is maybe right on Win32 (for this roaming
vs. local directory logics), but not on Unix-like systems, where we end
up trying to write in the data directory (usually not even supposed to
be writable by applications).
Also while at it, I replace g_get_prgname() by PACKAGE_NAME. It turns
out that this function returns NULL, maybe because of the init order.

Actually ideally, this file should rather go under:
$XDG_CACHE_HOME/GIMP/<version>/
But last we discussed this, it was decided that files should not spread
too much (though I still disagree!).