* configure.in: Enable internal WITH_BRIDGE support not just for LXC or
QEMU, but also for --with-network.  Russell Haering reported that
compilation failed when configured with --without-lxc --without-qemu.
Details: http://thread.gmane.org/gmane.comp.emulators.libvirt/11928
* src/bridge.h: Use #if WITH_BRIDGE, rather than currently-equivalent
"#if defined(WITH_QEMU) || defined(WITH_LXC)"