Pass --without-libproxy to neon build system

We neither build nor use libproxy, but autodetection will sometimes
build support for it when the host has it installed, which breaks the
build process because proxy.h cannot be found. We disable it explicitly
to prevent this.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Merge pull request #333 from ryao/build

Pass --without-libproxy to neon build system