autogen.sh: fail faster/clearer if g-i not found
autogen.sh: fail faster/clearer if g-i not found

GOBJECT_INTROSPECTION_CHECK macro can be missing if introspection
is not installed, so this way the following error is prevented:

"gtk/Makefile.am:1324: error: HAVE_INTROSPECTION does not appear
in AM_CONDITIONAL"

https://bugzilla.gnome.org/show_bug.cgi?id=723438

Lyude/gtk-,jadahl/gtk,Adamovskiy/gtk,chergert/gtk,Adamovskiy/gtk,jessevdk/gtk,ahodesuka/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk,Lyude/gtk-,jadahl/gtk,alexlarsson/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,alexlarsson/gtk,davidgumberg/gtk,jessevdk/gtk,grubersjoe/adwaita,Lyude/gtk-,jigpu/gtk,msteinert/gtk,alexlarsson/gtk,jigpu/gtk,alexlarsson/gtk,jadahl/gtk,Lyude/gtk-,jessevdk/gtk,chergert/gtk,davidgumberg/gtk,jigpu/gtk,grubersjoe/adwaita,ahodesuka/gtk,alexlarsson/gtk,grubersjoe/adwaita,msteinert/gtk,ahodesuka/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,Adamovskiy/gtk,davidgumberg/gtk,jadahl/gtk,Lyude/gtk-,Adamovskiy/gtk,davidgumberg/gtk,jadahl/gtk,chergert/gtk,Lyude/gtk-,jessevdk/gtk,ahodesuka/gtk,jigpu/gtk,chergert/gtk,Lyude/gtk-,grubersjoe/adwaita,jigpu/gtk,jessevdk/gtk,msteinert/gtk,alexlarsson/gtk,davidgumberg/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,davidgumberg/gtk,Adamovskiy/gtk,Adamovskiy/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,chergert/gtk,davidgumberg/gtk,msteinert/gtk,chergert/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,ahodesuka/gtk,msteinert/gtk,jessevdk/gtk,jadahl/gtk,chergert/gtk,chergert/gtk,davidgumberg/gtk,ahodesuka/gtk,jadahl/gtk,Lyude/gtk-,Adamovskiy/gtk,msteinert/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,alexlarsson/gtk