nginx: do not error out on warning

This pull request disables "-Werror" when building nginx. Our custom CPI needs Precise, so that's what our Bosh stemcells use. On Precise, the compiler warns about two unused variables, and "-Werror" makes the build error out.

Up to cf 155, we haven't been building nginx at all, and been setting the ccng.use_nginx flag to false. When that flag disappeared with 3343556d, we disabled "-Werror" instead. This shouldn't hurt when building on Lucid, so we're hoping you will approve this pull request.

Jon Kåre Hellan, UNINETT AS, Trondheim, Norway