zmq: switch to building with CMake

Faster compilation.

Before:

time make package/zmq/compile -j 12
Executed in   24.98 secs   fish           external
   usr time   97.41 secs  263.00 micros   97.40 secs
   sys time   12.51 secs   34.00 micros   12.51 secs

After:

time make package/zmq/compile -j 12
Executed in   18.17 secs   fish           external
   usr time   85.22 secs  248.00 micros   85.22 secs
   sys time    9.23 secs   32.00 micros    9.23 secs

Signed-off-by: Rosen Penev <rosenp@gmail.com>