Workaround for too old GL header on older SDL versions

Problem: Since the removal of !_IRR_OPENGL_USE_EXTPOINTER_ we require some quite recent
symbols to compile the legacy GL driver. (*) Since the previous commit we prefer including
them via SDL but those are sometimes too old.

(*) This was in fact always a problem since USE_EXTPOINTER is the default.
    I guess people just had recent enough headers usually.

source/Irrlicht/CMakeLists.txt

@@ -185,7 +185,7 @@ endif()
 
 if(ENABLE_OPENGL)
 	add_definitions(-D_IRR_COMPILE_WITH_OPENGL_)
-	set(OPENGL_DIRECT_LINK TRUE) # not yet possible to remove this
+	set(OPENGL_DIRECT_LINK TRUE) # driver relies on this
 	if(DEVICE STREQUAL "WINDOWS")
 		add_definitions(-D_IRR_COMPILE_WITH_WGL_MANAGER_)
 	elseif(DEVICE STREQUAL "X11")
@@ -269,6 +269,24 @@ if(USE_SDL2)
 	message(STATUS "Found SDL2: ${SDL2_LIBRARIES}")
 endif()
 
+# More special config
+
+if(ENABLE_OPENGL AND DEVICE STREQUAL "SDL")
+	# The legacy GL driver requires some symbols from GL 4.5 to compile,
+	# which SDL only provides since 2.26.0 (Nov 2022).
+	# We have a fallback in case this isn't satisfied so test for it.
+	set(CMAKE_REQUIRED_INCLUDES ${SDL2_INCLUDE_DIRS})
+	unset(CHECK_GL_VERSION_4_5 CACHE)
+	check_cxx_source_compiles("#include <SDL_opengl.h>\n\
+#ifndef GL_VERSION_4_5\n\
+#error\n\
+#endif\n\
+int main() {}" CHECK_GL_VERSION_4_5)
+	if(CHECK_GL_VERSION_4_5)
+		add_definitions(-DIRR_PREFER_SDL_GL_HEADER)
+	endif()
+endif()
+
 # Platform-specific libs
 
 if(ANDROID)

source/Irrlicht/COpenGLCommon.h

@@ -6,7 +6,7 @@
 
 #ifdef _IRR_COMPILE_WITH_OPENGL_
 
-#if defined(_IRR_COMPILE_WITH_SDL_DEVICE_)
+#if defined(_IRR_COMPILE_WITH_SDL_DEVICE_) && defined(IRR_PREFER_SDL_GL_HEADER)
 	#include <SDL_video.h>
 	#include <SDL_opengl.h>
 #else