I noticed while working on my current project that ALLEGRO_FILECHOOSER_FOLDER opens the normal file selector instead of the folder selector on Linux. I've attached a small patch which checks for the ALLEGRO_FILECHOOSER_FOLDER flag and sets the appropriate GTK FILE_CHOOSER_ACTION for folders so this works as expected.

Bumping as I completely forgot about this. Will take a look tonight.

EDIT: submitted.