build: make headers check compatible with distcc
build: make headers check compatible with distcc

distcc doesn't like the -x c++ flag, so create an empty.cc file for
this purpose and compile it.

Also drop the "=" from "--include=", which is also disliked by
distcc.
Message-Id: <6263c08260252f73f250f58477bbcf14746e1141@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla