Bother those kernel developers.  In the latest rawhide, kernel
and glibc have now been unified so that <netinet/in.h> and
<linux/in6.h> no longer clash; but <linux/if_bridge.h> is still
not self-contained.  Because of the latest header change, the
build is failing with:

checking for linux/param.h... no
configure: error: You must install kernel-headers in order to compile libvirt with QEMU or LXC support

with details:

In file included from conftest.c:561:0:
/usr/include/linux/in6.h:71:18: error: field 'flr_dst' has incomplete type
  struct in6_addr flr_dst;

We need a workaround to avoid our workaround :)

* configure.ac (NETINET_LINUX_WORKAROUND): New test.
* src/util/virnetdevbridge.c (includes): Use it.
Signed-off-by: NEric Blake <eblake@redhat.com>