Fix FetchContent Bullet with MSVC

extern/CMakeLists.txt

@@ -35,9 +35,13 @@ if(NOT OPENMW_USE_SYSTEM_BULLET)
         set(BUILD_SHARED_LIBS OFF CACHE BOOL "" FORCE)
     else()
         set(BUILD_SHARED_LIBS ON CACHE BOOL "" FORCE)
-        if(MSVC)
-            set(USE_MSVC_RUNTIME_LIBRARY_DLL ON CACHE BOOL "" FORCE)
-        endif()
+    endif()
+
+    if(MSVC)
+        # this setting is badly named - having it off forces the static runtime library,
+        # but having it on does nothing, letting the defaults get used.
+        # OpenMW uses the defaults, and you can't mix and match.
+        set(USE_MSVC_RUNTIME_LIBRARY_DLL ON CACHE BOOL "" FORCE)
     endif()
 
     # master on 12 Mar 2021