FindAzmqLibzmq: Avoid using ALIAS targets

When using target_link_libraries, the ALIAS target is resolved. So the
generated azmqTargets.cmake can end up with something like

    set_target_properties(Azmq::azmq PROPERTIES
      ...
      INTERFACE_LINK_LIBRARIES "PkgConfig::LIBZMQ;...

making the

    include(${CMAKE_CURRENT_LIST_DIR}/FindAzmqLibzmq.cmake)

in azmqConfig.cmake meaningless. We want that PkgConfig::LIBZMQ to stay
as Azmq::libzmq, so it can be resolved to the correct target at use
time.

Author: reddwarf69

FindAzmqLibzmq.cmake

@@ -9,12 +9,17 @@
 if (TARGET Azmq::libzmq)
     # allow overriding from the calling script
     message(STATUS "using provided Azmq::libzmq target")
-elseif (TARGET libzmq AND BUILD_SHARED_LIBS)
+    return()
+else()
+    add_library(Azmq::libzmq INTERFACE IMPORTED)
+endif()
+
+if (TARGET libzmq AND BUILD_SHARED_LIBS)
     message(STATUS "using existing shared library target")
-    add_library(Azmq::libzmq ALIAS libzmq)
+    target_link_libraries(Azmq::libzmq ALIAS libzmq)
 elseif (TARGET libzmq-static AND NOT BUILD_SHARED_LIBS)
     message(STATUS "using existing static library target")
-    add_library(Azmq::libzmq ALIAS libzmq-static)
+    target_link_libraries(Azmq::libzmq INTERFACE libzmq-static)
 else ()
     # try finding the package
     find_package(ZeroMQ QUIET)
@@ -24,14 +29,14 @@ else ()
         if (BUILD_SHARED_LIBS)
             if (TARGET libzmq)
                 message(STATUS "using cmake config libzmq")
-                add_library(Azmq::libzmq ALIAS libzmq)
+                target_link_libraries(Azmq::libzmq INTERFACE libzmq)
             else ()
                 message(FATAL_ERROR "libzmq not exported in the cmake configuration")
             endif ()
         else ()
             if (TARGET libzmq-static)
                 message(STATUS "using cmake config static libzmq")
-                add_library(Azmq::libzmq ALIAS libzmq-static)
+                target_link_libraries(Azmq::libzmq INTERFACE libzmq-static)
             else ()
                 message(FATAL_ERROR "libzmq-static not exported in the cmake configuration")
             endif ()
@@ -42,7 +47,7 @@ else ()
         find_package(PkgConfig REQUIRED)
         pkg_check_modules(LIBZMQ REQUIRED IMPORTED_TARGET GLOBAL libzmq)
         if (LIBZMQ_FOUND)
-            add_library(Azmq::libzmq ALIAS PkgConfig::LIBZMQ)
+            target_link_libraries(Azmq::libzmq INTERFACE PkgConfig::LIBZMQ)
         else ()
             message(FAIL_ERROR "Can't find the required libzmq library")
         endif ()