config: Pass the compiler -Werror during warning detection

config: Pass the compiler -Werror during warning detection

Otherwise the compilation prints a warning but exits with 0 return code.
However, later during the compilation, when -Werror is enforced to about
every file, the unsupported options start breaking the builds.

Tested: Ran configure with clang-3.5 and built libelf/ tree with it.

  $ autoreconf -i
  $ ./configure CC=clang-3.5 ...
  $ make -C libelf

Signed-off-by: Filipe Brandenburger <filbranden@google.com>