lib/flex_array: make build optional

Cc: Changli Gao <xiaosuo@gmail.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Eric Paris <eparis@redhat.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Jesse Gross <jesse@nicira.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Pravin Shelar <pshelar@nicira.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
lib/halfmd4: make build optional

Cc: Jan Kara <jack@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
lib/rhashtable: make build optional

Cc: David S. Miller <davem@davemloft.net>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Thomas Graf <tgraf@suug.ch>
Cc: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>