exo: add native build

The native build creates exo-csource only. exo-csource converts glade-files
into c-header files. When building xfce projects from git repo, exo-csource is
required (tarballs have the glade-headers included).

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>