lib/smbreadline: detect picky compile issue with readline.h

lib/smbreadline: detect picky compile issue with readline.h

readline.h has build issues with clang if -Wstrict-prototypes
is enabled. Detect this and also detect whether the known
workaround works.

Fix suggested by Timur I. Bakeyev <timur@freebsd.org>

cf. https://lists.gnu.org/archive/html/bug-readline/2014-04/msg00018.html
cf. https://lists.samba.org/archive/samba-technical/2017-November/123923.html

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>