build: Build with system md5.h on OpenIndiana

This changes (again...) our system md5 detection to cope with how
OpenIndiana does md5.  I'm becoming increasingly convinced this isn't
worth our while (we should have just done samba_md5...), but for now
this change seems to work on FreeBSD, OpenIndiana and Linux with
libbsd.

This needs us to rename struct MD5Context -> MD5_CTX, but we provide a
config.h define to rename the type bad if MD5_CTX does not exist (it does
however exist in the md5.h from libbsd).

Andrew Bartlett

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Jun 19 21:32:36 CEST 2013 on sn-devel-104