CMake: don't try to link with librt on MinGW

On MinGW, librt will might will found as rt library in MSYS, but fails linking with "cannot find -lrt" error.
As librt is not avaiable on native Windows, we should never try to link with -lrt on MinGW (which is native Windows).

CMakeLists.txt

@@ -362,9 +362,11 @@ else()
   check_cxx_symbol_exists(LOCAL_PEERCRED sys/socket.h ZMQ_HAVE_LOCAL_PEERCRED)
 endif()
 
-find_library(RT_LIBRARY rt)
-if(RT_LIBRARY)
-  set(pkg_config_libs_private "${pkg_config_libs_private} -lrt")
+if(NOT MINGW)
+  find_library(RT_LIBRARY rt)
+  if(RT_LIBRARY)
+    set(pkg_config_libs_private "${pkg_config_libs_private} -lrt")
+  endif()
 endif()
 
 find_package(Threads)