iproute2: fix build with toolchains providing setns(2)

iproute2: fix build with toolchains providing setns(2)

Fixes http://autobuild.buildroot.net/results/b2f28621bc9d6048057c406048b3f7d665cef165

ipnetns.c contains a replacement setns implementation, which is used
if the toolchain doesn't provide any (HAVE_SETNS not set).

We don't have any knowledge of toolchain setns support on buildroot
level, but the (handwritten) configure script contains a test for it,
so run the configure script before building.

The configure script isn't written for cross compilation, so it needs
to be massaged slightly to use the cross compiler / flags.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>