Add --with-libevent-dir and --with-openssl-dir args, instead of using env. vars.

rsavoye has prepared a patch for obfsproxy that introduces --with-libevent-lib and --with-libcrypto-lib, that do the job of '{libevent,libcrypto}_{LIBS,CFLAGS}.

note: maybe we should rename those options to match with tor's (that is, --with-libevent-dir and --with-openssl-dir).