cmake: rpath take 2, re previous commit [eea16e], use global

property `ZINT_LIBDIR` hack to set in both CLI and GUI, whether
  macOS or not (ticket #346, props Hagen Röwer and Bryce Harrison)

CMakeLists.txt

@@ -147,6 +147,9 @@ if(NOT HAVE_GETOPT_LONG_ONLY)
     add_subdirectory(getopt)
 endif()
 
+# For setting rpath in frontend and frontend_qt
+set_property(GLOBAL PROPERTY ZINT_LIBDIR "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
+
 add_subdirectory(backend)
 if(ZINT_FRONTEND)
     add_subdirectory(frontend)