The problem is that the new byte-ordering macros adopted on master
don't support CentOS 5 because they assumed that any Linux system had
endian(3) support. CentOS 6 (and presumably newer) do, but CentOS 5
doesn't.
So instead we only do glibc endian(3) support if we're on a system
with glibc 2.9 or higher (which is when this functionality was
introduced).
For any other platform that we don't detect (which now includes older
glibc such as CentOS 5), bring back our homebrewed htonll and ntohll
implementation from iperf 3.0.x.
Fixes#224.
eb9a2c07c3