The sources for usbip are within the kernel.  A patch that was included
with the package, which changed the old signal name SIGCLD to the new
one, SIGCHLD, was merged upstream.  However, different targets use
different kernel versions.  Current version 4.14 and 4.9 are fine, but
older versions do not have the patch applied.  So, I used
-DSIGCLD=SIGCHLD to please both worlds.

libudev-fbsd currently used by openwrt does not implement the
udev_device_get_devpath function.  eudev's implementation of libudev
sets it as (src/libudev/libudev-device.c):
udev_device->devpath = udev_device->syspath + strlen("/sys");
I used a command-line define to use the same logic, as it works with
new and old versions of the kernel--the use of ..devpath is quite
recent.

I also linked with libbsd, when using glibc.

Signed-off-by: Eneas U de Queiroz <cote2004-github@yahoo.com>