Depending on non-doc targets being built before and the setting of DESTDIR
the copy of the examples dir being part of install-doc could in some cases
fail with a non existent "$(DESTDIR)$(datadir)" target directory.
Add the conditional rte_mkdir for that to avoid the issue.
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com> Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>