That should solve link-time problems when building with mingw.

Not supplying "SUBSYSTEM:" flags to mingw linker.
Linking mygui with libwinmm - should resolve reference to timeGetTime().

CMakeLists.txt

@@ -308,6 +308,7 @@ if (BUILD_ESMTOOL)
 endif()
 
 if (WIN32)
+  if (MSVC)
     if (USE_DEBUG_CONSOLE)
       set_target_properties(openmw PROPERTIES LINK_FLAGS_DEBUG "/SUBSYSTEM:CONSOLE")
       set_target_properties(openmw PROPERTIES LINK_FLAGS_RELWITHDEBINFO "/SUBSYSTEM:CONSOLE")
@@ -322,6 +323,7 @@ if (WIN32)
     set_target_properties(openmw PROPERTIES LINK_FLAGS_RELEASE "/SUBSYSTEM:CONSOLE")
     set_target_properties(openmw PROPERTIES COMPILE_DEFINITIONS_RELEASE "_CONSOLE")
     set_target_properties(openmw PROPERTIES LINK_FLAGS_MINSIZEREL "/SUBSYSTEM:CONSOLE")
+  endif(MSVC)
   
   # TODO: At some point release builds should not use the console but rather write to a log file
   #set_target_properties(openmw PROPERTIES LINK_FLAGS_RELEASE "/SUBSYSTEM:WINDOWS")

extern/mygui_3.0.1/MyGUIEngine/CMakeLists.txt

@@ -26,3 +26,7 @@ if (MYGUI_USE_FREETYPE)
 endif()
 
 target_link_libraries(${PROJECT_NAME} ${CMAKE_DL_LIBS} uuid)
+
+if (MINGW)
+  target_link_libraries(${PROJECT_NAME} libwinmm.a)
+endif (MINGW)