fix install without static libzmq

If libzmq is only available as a shared library and not a static one
then cmake fails with:

-- CMake libzmq package not found, trying again with pkg-config (normal install of zeromq)
-- Found PkgConfig: /home/fabrice/buildroot/output/host/bin/pkg-config (found version "0.28")
CMake Error at CMakeLists.txt:20 (message):
  ZeroMQ was not found, neither as a CMake package nor via pkg-config

This is due to the fact that ZeroMQ_FOUND is not set to ON even if
ZEROMQ_LIBRARY is TRUE:

if(ZeroMQ_LIBRARY AND ZeroMQ_STATIC_LIBRARY)
    set(ZeroMQ_FOUND ON)
endif()

So change AND by OR as suggested in
#266

Fix #266

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

libzmq-pkg-config/FindZeroMQ.cmake

@@ -8,7 +8,7 @@ find_library(ZeroMQ_LIBRARY NAMES libzmq.so libzmq.dylib libzmq.dll
 find_library(ZeroMQ_STATIC_LIBRARY NAMES libzmq-static.a libzmq.a libzmq.dll.a
              PATHS ${PC_LIBZMQ_LIBDIR} ${PC_LIBZMQ_LIBRARY_DIRS})
 
-if(ZeroMQ_LIBRARY AND ZeroMQ_STATIC_LIBRARY)
+if(ZeroMQ_LIBRARY OR ZeroMQ_STATIC_LIBRARY)
     set(ZeroMQ_FOUND ON)
 endif()