moved zmq function to new file

cmake/SlsFindZeroMQ.cmake

@@ -0,0 +1,36 @@
+function(custom_find_zmq)
+    set(ZeroMQ_HINT "" CACHE STRING "Hint where ZeroMQ could be found")
+    #Adapted from: https://github.com/zeromq/cppzmq/
+    if (NOT TARGET libzmq)
+        if(ZeroMQ_HINT)
+            message(STATUS "Looking for ZeroMQ in: ${ZeroMQ_HINT}")
+            find_package(ZeroMQ 4 
+                NO_DEFAULT_PATH
+                HINTS ${ZeroMQ_DIR}
+            )
+        else()
+            find_package(ZeroMQ 4 QUIET)
+        endif()
+        
+    # libzmq autotools install: fallback to pkg-config
+    if(NOT ZeroMQ_FOUND)
+        message(STATUS "CMake libzmq package not found, trying again with pkg-config (normal install of zeromq)")
+        list (APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/cmake/libzmq-pkg-config)
+        find_package(ZeroMQ 4 REQUIRED)
+    endif()
+
+    # TODO "REQUIRED" above should already cause a fatal failure if not found, but this doesn't seem to work
+    if(NOT ZeroMQ_FOUND)
+        message(FATAL_ERROR "ZeroMQ was not found, neither as a CMake package nor via pkg-config")
+    endif()
+
+    if (ZeroMQ_FOUND AND NOT TARGET libzmq)
+        message(FATAL_ERROR "ZeroMQ version not supported!")
+    endif()
+    endif()
+
+    get_target_property(VAR libzmq IMPORTED_LOCATION)
+    message(STATUS "Using zmqlib: ${VAR}")
+
+
+endfunction()