build: fix configure detection of if_bridge.h on RHEL 6

A fresh checkout on a RHEL 6 machine with these packages:
kernel-headers-2.6.32-405.el6.x86_64
glibc-2.12-1.128.el6.x86_64
failed to configure with this message:
checking for linux/if_bridge.h... no
configure: error: You must install kernel-headers in order to compile libvirt with QEMU or LXC support

Digging in config.log, we see that the problem is identical to
what we fixed earlier in commit d12c2811:

configure:98831: checking for linux/if_bridge.h
configure:98853: gcc -std=gnu99 -c -g -O2  conftest.c >&5
In file included from /usr/include/linux/if_bridge.h:17,
                 from conftest.c:559:
/usr/include/linux/in6.h:31: error: redefinition of 'struct in6_addr'
/usr/include/linux/in6.h:48: error: redefinition of 'struct sockaddr_in6'
/usr/include/linux/in6.h:56: error: redefinition of 'struct ipv6_mreq'
configure:98860: $? = 1

I had not hit it earlier because I was using incremental builds,
where config.cache had shielded me from the kernel-headers breakage.

* configure.ac (if_bridge.h): Avoid conflicting type definitions.
Signed-off-by: NEric Blake <eblake@redhat.com>

configure.ac

@@ -997,7 +997,14 @@ if test "$with_linux" = "yes"; then
   if test "$with_qemu" = "yes" || test "$with_lxc" = "yes" ; then
     AC_CHECK_HEADERS([linux/param.h linux/sockios.h linux/if_bridge.h linux/if_tun.h],,
       [AC_MSG_ERROR([You must install kernel-headers in order to compile libvirt with QEMU or LXC support])],
-      [[#include <netinet/in.h>
+      [[/* The kernel folks broke their headers when used with particular
+         * glibc versions; although the structs are ABI compatible, the
+         * C type system doesn't like struct redefinitions.  We work around
+         * the problem here in the same manner as in virnetdevbridge.c.  */
+        #include <netinet/in.h>
+        #define in6_addr in6_addr_
+        #define sockaddr_in6 sockaddr_in6_
+        #define ipv6_mreq ipv6_mreq_
       ]])
   fi
 fi