MSVC: extern/ tweaks to make it build

Not everything is supported but it does build with the following CMakeSettings.json
variables and dependencies from vcpkg:

      "variables": [
        { "name": "OPENMW_USE_SYSTEM_BULLET", "value": "False", "type": "BOOL" },
        { "name": "OPENMW_USE_SYSTEM_MYGUI", "value": "False", "type": "BOOL" },
        { "name": "OPENMW_USE_SYSTEM_OSG", "value": "False", "type": "BOOL" },
        { "name": "BULLET_STATIC", "value": "True", "type": "BOOL" },
        { "name": "OSG_STATIC", "value": "False", "type": "BOOL" },
        { "name": "MYGUI_STATIC", "value": "False", "type": "BOOL" }
      ],

What works: it builds

What does not work: Not all DLLs are copied into the output directory with this set up
(SDL2, MyGUI, Bullet, OSG, are not copied).

CMakeLists.txt

@@ -115,6 +115,12 @@ option(SDL2_STATIC "Link static build of SDL into the binaries" FALSE)
 option(QT_STATIC "Link static build of QT into the binaries" FALSE)
 
 option(OPENMW_USE_SYSTEM_BULLET "Use system provided bullet physics library" ON)
+if(OPENMW_USE_SYSTEM_BULLET)
+    set(_bullet_static_default OFF)
+else()
+    set(_bullet_static_default ON)
+endif()
+option(BULLET_STATIC "Link static build of Bullet into the binaries" ${_bullet_static_default})
 
 option(OPENMW_USE_SYSTEM_OSG "Use system provided OpenSceneGraph libraries" ON)
 if(OPENMW_USE_SYSTEM_OSG)

extern/CMakeLists.txt

@@ -31,6 +31,15 @@ if(NOT OPENMW_USE_SYSTEM_BULLET)
     set(USE_DOUBLE_PRECISION ${BULLET_USE_DOUBLES} CACHE BOOL "")
     set(BULLET2_MULTITHREADING ON CACHE BOOL "")
 
+    if(BULLET_STATIC)
+        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()
+
     # master on 12 Mar 2021
     include(FetchContent)
     FetchContent_Declare(bullet
@@ -61,9 +70,9 @@ if(NOT OPENMW_USE_SYSTEM_MYGUI)
     set(MYGUI_DONT_USE_OBSOLETE OFF CACHE BOOL "")
 
     if(MYGUI_STATIC)
-        set(BUILD_SHARED_LIBS OFF)
+        set(BUILD_SHARED_LIBS OFF CACHE BOOL "" FORCE)
     else()
-        set(BUILD_SHARED_LIBS ON)
+        set(BUILD_SHARED_LIBS ON CACHE BOOL "" FORCE)
     endif()
 
     include(FetchContent)
@@ -81,8 +90,6 @@ endif()
 if(NOT OPENMW_USE_SYSTEM_OSG)
     cmake_minimum_required(VERSION 3.11) # for FetchContent
 
-    set(DYNAMIC_OPENTHREADS OFF CACHE BOOL "")
-    set(DYNAMIC_OPENSCENEGRAPH OFF CACHE BOOL "")
     set(BUILD_OSG_APPLICATIONS OFF CACHE BOOL "")
     set(BUILD_OSG_DEPRECATED_SERIALIZERS OFF CACHE BOOL "")
     set(OSG_FIND_3RD_PARTY_DEPS OFF CACHE BOOL "")
@@ -104,9 +111,33 @@ if(NOT OPENMW_USE_SYSTEM_OSG)
     set(OPENGL_PROFILE "GL2" CACHE STRING "")
 
     if(OSG_STATIC)
-        set(BUILD_SHARED_LIBS OFF)
+        set(BUILD_SHARED_LIBS OFF CACHE BOOL "" FORCE)
+        set(DYNAMIC_OPENTHREADS OFF CACHE BOOL "" FORCE)
+        set(DYNAMIC_OPENSCENEGRAPH OFF CACHE BOOL "" FORCE)
     else()
-        set(BUILD_SHARED_LIBS ON)
+        set(BUILD_SHARED_LIBS ON CACHE BOOL "" FORCE)
+        set(DYNAMIC_OPENTHREADS ON CACHE BOOL "" FORCE)
+        set(DYNAMIC_OPENSCENEGRAPH ON CACHE BOOL "" FORCE)
+    endif()
+    mark_as_advanced(DYNAMIC_OPENTHREADS DYNAMIC_OPENSCENEGRAPH)
+
+    if(WIN32)
+        # OSG here inherits C++17 language level because it doesn't specify its own.
+        #
+        # OSG's `using namespace std` interferes with Windows header files.
+        #
+        # See https://developercommunity.visualstudio.com/t/error-c2872-byte-ambiguous-symbol/93889
+        #
+        # An alternative way to work around this without changing the language level is:
+        #
+        # add_compile_definitions(_HAS_STD_BYTE=0)
+        #
+        # TODO: Put OSG into its own scope so that this does not leak into Recast below.
+        set(CMAKE_CXX_STANDARD 11)
+
+        if(MSVC)
+            set(OSG_MSVC_VERSIONED_DLL OFF CACHE BOOL "")
+        endif()
     endif()
 
     # branch OpenSceneGraph-3.6 on 23 Jan 2021.