perf tools: Add support for disabling -Werror via WERROR=0

perf tools: Add support for disabling -Werror via WERROR=0

GCC often introduces new warnings with lots of false positives -
breaking -Werror builds. WERROR=0 allows one to build perf without much
fuss - while still encouraging people to send patches to avoid the fuss
of having to type WERROR=0.

Bisecting back to commits that produce a (mostly harmless) warning on
some compilers is more difficult. With WERROR=0 one could bisect without
worrying about harmless warnings.

Cc: Ingo Molnar <mingo@elte.hu>
Link: http://lkml.kernel.org/r/eac06c7cc4920e5d4830417d466161fb26c7359c.1315514559.git.dvhart@linux.intel.com
Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>