gnome-autogen.sh: correct IFS value

Due to a likely copy-paste mistake, the default value of IFS was being
set to 9 spaces instead of a tab and a space.  Fix that.

https://bugzilla.gnome.org/show_bug.cgi?id=726208
gnome-common: use ACLOCAL_PATH over ACLOCAL_FLAGS

jhbuild has set this variable for more than two years.

https://bugzilla.gnome.org/show_bug.cgi?id=726208