make use of CONFIG and also put in TODO

CMakeLists.txt

@@ -366,7 +366,7 @@ endif()
 # CXXOPTS
 option(OPENMW_USE_SYSTEM_CXXOPTS "Use system provided cxxopts library" ON)
 if(OPENMW_USE_SYSTEM_CXXOPTS)
-    find_package(cxxopts REQUIRED)
+    find_package(cxxopts CONFIG REQUIRED)
 endif()
 
 # Platform specific

extern/CMakeLists.txt

@@ -202,6 +202,7 @@ if(NOT OPENMW_USE_SYSTEM_CXXOPTS)
     ### here is what makes all include directories -isystem
     get_target_property(CXXOPTS_IID cxxopts INTERFACE_INCLUDE_DIRECTORIES)
     set_target_properties(cxxopts PROPERTIES INTERFACE_SYSTEM_INCLUDE_DIRECTORIES "${CXXOPTS_IID}")
+    ### TODO: use SYSTEM option to FetchContent_Declare once CMake 3.25 is available and remove the above workaround
 endif()
 
 if (NOT OPENMW_USE_SYSTEM_SQLITE3)