iputils: disable on older uclibc versions

Disable when using a uclibc version before 0.9.33 since dn_comp
function support was added in this version. Also disabling on AVR32
since any AVR32 toolchain will be based on a uclibc version older
than 0.9.33 (for using an external AVR32 toolchain).

[Peter: use dn_comp instead of __dn_comp]
Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>