Cleaning up the find zmq (#431)

cmake/SlsFindZeroMQ.cmake

@@ -0,0 +1,38 @@
+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(ZeroMQ_FOUND)
+        message(STATUS "Found libzmq using find_package")
+    else()
+        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 libzmq: ${VAR}")
+
+
+endfunction()

cmake/libzmq-pkg-config/FindZeroMQ.cmake

@@ -0,0 +1,28 @@
+#From: https://github.com/zeromq/cppzmq/
+set(PKG_CONFIG_USE_CMAKE_PREFIX_PATH ON)
+find_package(PkgConfig)
+pkg_check_modules(PC_LIBZMQ QUIET libzmq)
+
+set(ZeroMQ_VERSION ${PC_LIBZMQ_VERSION})
+find_library(ZeroMQ_LIBRARY NAMES libzmq.so libzmq.dylib libzmq.dll
+             PATHS ${PC_LIBZMQ_LIBDIR} ${PC_LIBZMQ_LIBRARY_DIRS})
+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 OR ZeroMQ_STATIC_LIBRARY)
+    set(ZeroMQ_FOUND ON)
+    message(STATUS "Found libzmq using PkgConfig")
+endif()
+
+if (TARGET libzmq)
+    # avoid errors defining targets twice
+    return()
+endif()
+
+add_library(libzmq SHARED IMPORTED)
+set_property(TARGET libzmq PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${PC_LIBZMQ_INCLUDE_DIRS})
+set_property(TARGET libzmq PROPERTY IMPORTED_LOCATION ${ZeroMQ_LIBRARY})
+
+add_library(libzmq-static STATIC IMPORTED ${PC_LIBZMQ_INCLUDE_DIRS})
+set_property(TARGET libzmq-static PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${PC_LIBZMQ_INCLUDE_DIRS})
+set_property(TARGET libzmq-static PROPERTY IMPORTED_LOCATION ${ZeroMQ_STATIC_LIBRARY})