configure.in: Yet another attempt at fixing the mess that is AC_LANG_PROGRAM().
src/libcollectdclient/network_buffer.c check for htonll exitence
before defining it. I have to to this to get it compiled on AIX6

Signed-off-by: Florian Forster <octo@collectd.org>
Merge branch 'collectd-5.1' into collectd-5.2