depends: add ability to skip building zeromq

depends/packages/packages.mk

@@ -1,4 +1,4 @@
-packages:=boost openssl libevent zeromq
+packages:=boost openssl libevent
 
 qt_native_packages = native_protobuf
 qt_packages = qrencode protobuf zlib
@@ -12,6 +12,8 @@ qt_mingw32_packages=qt
 
 wallet_packages=bdb
 
+zmq_packages=zeromq
+
 upnp_packages=miniupnpc
 
 darwin_native_packages = native_biplist native_ds_store native_mac_alias