build: skip sub-directories containing disabled plugins

build: skip sub-directories containing disabled plugins

Currently, make enters all sub-directories containing source-code, even
if they only contain optional targets which are not configured to be
built.  Instead, change the Makefiles so that the sub-directories are
optional, rather than the targets.

Group sub-directory definitions consistently at the top of the Makefiles
that contain them.

Trim a few leading and trailing blank lines.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>