obfsproxy configure: error: could not find ntohl
obfsproxy autoconf has a weird habit of putting file names on the build line of test programs. For example, if you incorrectly set libevent_LIBS="/usr/local/lib" (without the -L) then autoconf will just plunk that down onto the build line of its program that tests whether ntohl works. The test program won't build ("why did you say /usr/local/lib on my command line"), so autoconf concludes there's no ntohl.
People seem to report the "could not find ntohl" for all sorts of reasons, each of which boil down to some other thing in their build environment going wrong, and the ntohl test being the first one in autoconf's list.