build: Require glib 2.38 for g_assert_true

https://bugzilla.gnome.org/show_bug.cgi?id=711776
extract: Save original filename

And not the lowercase filename. Fixes issues with case-mismatch

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=731678