samba4: fix readline support

Add automatic readline support since it's used when available.
Also add a patch to update to new-style typedefs that were removed from
readline 6.3 that causes build breakage. Fixes:
http://autobuild.buildroot.net/results/b13/b137c237ff6df81dd10f7895278d1f2f5d2326de/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>