autogen.sh: Fix --simple-install case (again)

Using --simple-install with autogen.sh was broken by commit
3d03dc2487d128b949cde5e8b3b5a06d9ab40f89 because it wrongly assumed that
0 was true and 1 was false, when the enable_autotools variable treated
them the other way around.

https://bugzilla.gnome.org/show_bug.cgi?id=708376