Make sure libwinpthread is static linked

Not sure why this fixes the lib not being static linked, nor do I know what the
problem was making it not be static linked.

CMakeLists.txt

@@ -344,15 +344,15 @@ else()
         option(ALSOFT_STATIC_WINPTHREAD "Static link libwinpthread" OFF)
         if(ALSOFT_STATIC_WINPTHREAD)
             set(OLD_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
-            set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} "-Wl,--push-state,-Bstatic,-lwinpthread,--pop-state")
+            set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} "-Wl,--push-state,-Bstatic,-lstdc++,-lwinpthread,--pop-state")
             check_cxx_source_compiles("int main() { }" HAVE_STATIC_LIBWINPTHREAD_SWITCH)
             set(CMAKE_REQUIRED_LIBRARIES ${OLD_REQUIRED_LIBRARIES})
             unset(OLD_REQUIRED_LIBRARIES)
 
             if(NOT HAVE_STATIC_LIBWINPTHREAD_SWITCH)
                 message(FATAL_ERROR "Cannot static link libwinpthread")
             endif()
-            set(LINKER_FLAGS ${LINKER_FLAGS} "-Wl,--push-state,-Bstatic,-lwinpthread,--pop-state")
+            set(LINKER_FLAGS ${LINKER_FLAGS} "-Wl,--push-state,-Bstatic,-lstdc++,-lwinpthread,--pop-state")
         endif()
     endif()
 endif()