build: warn about mixed declarations and code

Attempt to use the -Wdeclaration-after-statement cflag when building in
developer mode.

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Sat Feb  1 20:20:35 CET 2014 on sn-devel-104