cmake: Move MYGUI/OSG_STATIC default to top-level

Makes it clear that the USE_SYSTEM variables affect the defaults of
STATIC variables.

CMakeLists.txt

@@ -111,11 +111,17 @@ option(QT_STATIC "Link static build of QT into the binaries" FALSE)
 
 option(OPENMW_USE_SYSTEM_OSG "Use system provided OpenSceneGraph libraries" ON)
 option(OSG_STATIC "Link static build of OpenSceneGraph into the binaries" OFF)
+if(NOT OPENMW_USE_SYSTEM_OSG)
+    set(OSG_STATIC ON CACHE BOOL "")
+endif()
 
 option(OPENMW_USE_SYSTEM_BULLET "Use system provided bullet physics library" ON)
 
 option(OPENMW_USE_SYSTEM_MYGUI "Use system provided mygui library" ON)
 option(MYGUI_STATIC "Link static build of Mygui into the binaries" OFF)
+if(NOT OPENMW_USE_SYSTEM_MYGUI)
+    set(MYGUI_STATIC ON CACHE BOOL "")
+endif()
 
 option(OPENMW_UNITY_BUILD "Use fewer compilation units to speed up compile time" FALSE)
 option(OPENMW_LTO_BUILD "Build OpenMW with Link-Time Optimization (Needs ~2GB of RAM)" OFF)

extern/CMakeLists.txt

@@ -54,8 +54,6 @@ endif()
 if(NOT OPENMW_USE_SYSTEM_MYGUI)
     cmake_minimum_required(VERSION 3.11) # for FetchContent
 
-    set(MYGUI_STATIC ON CACHE BOOL "" PARENT_SCOPE)
-
     set(MYGUI_RENDERSYSTEM 4 CACHE STRING "")
     set(MYGUI_DISABLE_PLUGINS TRUE CACHE BOOL "")
     set(MYGUI_BUILD_DEMOS OFF CACHE BOOL "")
@@ -78,8 +76,6 @@ endif()
 if(NOT OPENMW_USE_SYSTEM_OSG)
     cmake_minimum_required(VERSION 3.11) # for FetchContent
 
-    set(OSG_STATIC ON CACHE BOOL "" PARENT_SCOPE)
-
     set(DYNAMIC_OPENTHREADS OFF CACHE BOOL "")
     set(DYNAMIC_OPENSCENEGRAPH OFF CACHE BOOL "")
     set(BUILD_OSG_APPLICATIONS OFF CACHE BOOL "")