czmq: unbreak static linking

czmq: unbreak static linking

Fixes:
http://autobuild.buildroot.net/results/faf/faf0bb00fccb1350afaef55f6cf14c11a5f5dc3c/
http://autobuild.buildroot.net/results/8d2/8d279baa4c8bffbb01f897119092c7b736942df4/
http://autobuild.buildroot.net/results/4d3/4d3dea604da9a5a1e7fe20548813f8de474ae33f/
http://autobuild.buildroot.net/results/fd6/fd602617839817352763e51754553960b7f795ac/

And many more.

Zeromq uses C++, but czmq forgets to explicitly link with libstdc++
(or use g++ to link), breaking static linking.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>