czmq: prevent selection when ADI Blackfin external toolchains are used

czmq fails to build with the Blackfin external toolchains provided by
Analog Devices, because their uClibc configuration doesn't include
AI_ADDRCONFIG. In order to prevent such build failures to happen, we
simply prevent the selection of czmq or one of its reverse
dependencies when such toolchains are used.

This fixes:

  http://autobuild.buildroot.org/results/821/82140fac4c2a2cef3f38f06cada8f17fd7f0078b/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>