Switch Irrlicht dependency to our own fork

-> https://github.com/minetest/irrlicht

CMakeLists.txt

@@ -61,6 +61,29 @@ set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")
 
 # This is done here so that relative search paths are more reasonable
 find_package(Irrlicht)
+if(BUILD_CLIENT AND NOT IRRLICHT_FOUND)
+	message(FATAL_ERROR "Irrlicht is required to build the client, but it was not found.")
+elseif(IRRLICHT_INCLUDE_DIR STREQUAL "")
+	message(FATAL_ERROR "Irrlicht headers are required to build the server, but none found.")
+endif()
+
+include(CheckSymbolExists)
+set(CMAKE_REQUIRED_INCLUDES ${IRRLICHT_INCLUDE_DIR})
+unset(HAS_FORKED_IRRLICHT CACHE)
+check_symbol_exists(IRRLICHT_VERSION_MT "IrrCompileConfig.h" HAS_FORKED_IRRLICHT)
+if(NOT HAS_FORKED_IRRLICHT)
+	string(CONCAT EXPLANATION_MSG
+		"Irrlicht found, but it is not Minetest's Irrlicht fork. "
+		"The Minetest team has forked Irrlicht to make their own customizations. "
+		"It can be found here: https://github.com/minetest/irrlicht")
+	if(BUILD_CLIENT)
+		message(FATAL_ERROR "${EXPLANATION_MSG}\n"
+			"Building the client with upstream Irrlicht is no longer possible.")
+	else()
+		message(WARNING "${EXPLANATION_MSG}\n"
+			"The server can still be built with upstream Irrlicht but this is DISCOURAGED.")
+	endif()
+endif()
 
 
 # Installation