Get rid of overengineered macro

While the macro's implementation was fine based on the documentation for CMake 3.16, which we (currently) use, later versions of CMake recommend against this approach.

https://gitlab.com/OpenMW/openmw/-/merge_requests/4777 has an implementation that picks between a native feature added later that does the same job if it's available, but uses the one we already had if it isn't.
That's not ideal as the implementation ends up complicated and we'd have two code paths to support.

There's a simpler approach where we just temporarily set the directory property and call the real function, which would probably be the best approach in the general case.

However, as we want to set the directory property for everything in the directory, we can just set it on the `extern` directory, and then we don't need the macro at all.

extern/CMakeLists.txt

@@ -1,22 +1,8 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 
+set_directory_properties(PROPERTIES EXCLUDE_FROM_ALL on)
 set(CMAKE_CXX_CLANG_TIDY "")
 
-# Like `FetchContent_MakeAvailable` but passes EXCLUDE_FROM_ALL to `add_subdirectory`.
-macro(FetchContent_MakeAvailableExcludeFromAll)
-    foreach(contentName IN ITEMS ${ARGV})
-        string(TOLOWER ${contentName} contentNameLower)
-        FetchContent_GetProperties(${contentName})
-        if(NOT ${contentNameLower}_POPULATED)
-            FetchContent_Populate(${contentName})
-            if(EXISTS ${${contentNameLower}_SOURCE_DIR}/CMakeLists.txt)
-                add_subdirectory(${${contentNameLower}_SOURCE_DIR}
-                    ${${contentNameLower}_BINARY_DIR} EXCLUDE_FROM_ALL)
-            endif()
-        endif()
-    endforeach()
-endmacro()
-
 if(NOT OPENMW_USE_SYSTEM_BULLET)
 
     set(BUILD_BULLET3 OFF CACHE BOOL "")
@@ -52,7 +38,7 @@ if(NOT OPENMW_USE_SYSTEM_BULLET)
         URL_HASH SHA512=a5105bf5f1dd365a64a350755c7d2c97942f74897a18dcdb3651e6732fd55cc1030a096f5808cf50575281f05e3ac09aa50a48d271a47b94cd61f5167a72b7cc
         SOURCE_DIR fetched/bullet
     )
-    FetchContent_MakeAvailableExcludeFromAll(bullet)
+    FetchContent_MakeAvailable(bullet)
 
     set(BULLET_INCLUDE_DIRS ${bullet_SOURCE_DIR}/src PARENT_SCOPE)
 
@@ -82,7 +68,7 @@ if(NOT OPENMW_USE_SYSTEM_MYGUI)
         URL_HASH SHA512=c804ef665e786076582367f171082cd2181fdbd214300ddcca4a4245c5a0f45e62e72778ee2d96ec46b393e22477dd729f9bb3006e6eecbf536674e34a057721
         SOURCE_DIR fetched/mygui
     )
-    FetchContent_MakeAvailableExcludeFromAll(mygui)
+    FetchContent_MakeAvailable(mygui)
 
     set(MyGUI_INCLUDE_DIRS ${mygui_SOURCE_DIR}/MyGUIEngine/include PARENT_SCOPE)
     set(MyGUI_LIBRARIES MyGUIEngine PARENT_SCOPE)
@@ -148,7 +134,7 @@ if(NOT OPENMW_USE_SYSTEM_OSG)
         URL_HASH SHA512=9b0a94c1c1d99c767f1857629d43c7a53bfcb74ef760993a121567831e168a1ebbfc10b0c67d7f2241c7b6c6dab2d0e6b876d9f17aca0fabe1a8c86b2279f956
         SOURCE_DIR fetched/osg
     )
-    FetchContent_MakeAvailableExcludeFromAll(osg)
+    FetchContent_MakeAvailable(osg)
 
     set(OPENSCENEGRAPH_INCLUDE_DIRS ${osg_SOURCE_DIR}/include ${osg_BINARY_DIR}/include PARENT_SCOPE)
     set(OSG_LIBRARIES OpenThreads osg PARENT_SCOPE)
@@ -179,7 +165,7 @@ if(NOT OPENMW_USE_SYSTEM_RECASTNAVIGATION)
         URL_HASH SHA512=48f20cee7a70c2f20f4c68bb74d5af11a1434be85294e37f5fe7b7aae820fbcdff4f35d3be286eaf6f9cbce0aed4201fcc090df409a5bd04aec5fd7c29b3ad94
         SOURCE_DIR fetched/recastnavigation
     )
-    FetchContent_MakeAvailableExcludeFromAll(recastnavigation)
+    FetchContent_MakeAvailable(recastnavigation)
 endif()
 
 if (NOT OPENMW_USE_SYSTEM_SQLITE3)
@@ -189,7 +175,7 @@ if (NOT OPENMW_USE_SYSTEM_SQLITE3)
         URL_HASH SHA512=7de291709e88fffc693cf24ac675950cfc35c1bf7631cfea95167105720a05cf37fb943c57c5c985db2eeaa57b31894b3c0df98a7bd2939b5746fc5a24b5ae87
         SOURCE_DIR fetched/sqlite3
     )
-    FetchContent_MakeAvailableExcludeFromAll(sqlite3)
+    FetchContent_MakeAvailable(sqlite3)
 
     add_library(sqlite3 STATIC ${sqlite3_SOURCE_DIR}/sqlite3.c)
     target_include_directories(sqlite3 INTERFACE ${sqlite3_SOURCE_DIR}/)
@@ -217,7 +203,7 @@ if (BUILD_BENCHMARKS AND NOT OPENMW_USE_SYSTEM_BENCHMARK)
         URL_HASH SHA512=d73587ad9c49338749e1d117a6f8c7ff9c603a91a2ffa91a7355c7df7dea82710b9a810d34ddfef20973ecdc77092ec10fb2b4e4cc8d2e7810cbed79617b3828
         SOURCE_DIR fetched/benchmark
     )
-    FetchContent_MakeAvailableExcludeFromAll(benchmark)
+    FetchContent_MakeAvailable(benchmark)
 endif()
 
 if (NOT OPENMW_USE_SYSTEM_YAML_CPP)
@@ -233,7 +219,7 @@ if (NOT OPENMW_USE_SYSTEM_YAML_CPP)
         URL_HASH MD5=1e8ca0d6ccf99f3ed9506c1f6937d0ec
         SOURCE_DIR fetched/yaml-cpp
     )
-    FetchContent_MakeAvailableExcludeFromAll(yaml-cpp)
+    FetchContent_MakeAvailable(yaml-cpp)
 
     if (MSVC AND PRECOMPILE_HEADERS_WITH_MSVC)
         target_precompile_headers(yaml-cpp PRIVATE <algorithm>)
@@ -306,7 +292,7 @@ if ((BUILD_COMPONENTS_TESTS OR BUILD_OPENCS_TESTS OR BUILD_OPENMW_TESTS) AND NOT
     if (MSVC)
         set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
     endif()
-    FetchContent_MakeAvailableExcludeFromAll(googletest)
+    FetchContent_MakeAvailable(googletest)
 
     add_library(GTest::GTest ALIAS gtest)
     add_library(GMock::GMock ALIAS gmock)