xfce-git: Run autogen.sh in source directory

The autogen.sh script executed by the xfce-git.bbclass needs to be
run in and from the source directory.  The configuration step can
still be run from out-of-tree.

Currently, this affects these recipes:
 * libxfce4ui
 * xarchiver
 * xfce4-taskmanager
 * xfce4-closebutton-plugin
 * xfce4-settings
 * xfwm4

Upstream-Status: Inappropriate (configuration)

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>