Modernize configure.ac

Replace the old, very long autogen.sh with a 3-line bootstrap.
Use Autoconf portable macros where applicable.

This also makes the bootstrap fail right away when trying to compile on
a system without libgtk/glib, instead of throwing an obscure error at
configure time.

Signed-off-by: Alexandre Montplaisir <alexandre.montplaisir@gmail.com>
Signed-off-by: Yannick Brosseau <yannick.brosseau@gmail.com>