package/zmqpp: Blacklist Blackfin ADI toolchains

zmqpp recommand at least a c++ compiler g++ >= 4.7
[1] for c++11 support and won't be backward compatible
whith older compliler for the next release (4.x)
due to harcoded -std=c++11 flag [2]

Blackfin ADI toolchains use a g++ 4.3 which use an
experimental c++11 support (c++0x) has not enough c++11
support to build zmqpp.

Fixes:
http://autobuild.buildroot.net/results/4c3/4c34ce881e3eab47994cc893898cbc9129ce67b5/

And many more.

[1] https://github.com/zeromq/zmqpp
[2] https://github.com/zeromq/zmqpp/commit/f078fe9a5a775aff5c74dedbdc869f8158ddf123



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>