tools lib subcmd: Respect WERROR=0 for build

this enables the workaround for compilers that generate warnings when
compiling libsubcmd.

Signed-off-by: Chris Phlipot <cphlipot0@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1467349955-1135-3-git-send-email-cphlipot0@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>