{cmake} Add GODOT_CPP_SYSTEM_HEADERS option to mark includes as SYSTEM

From the cmake docs:

"This may have effects such as suppressing warnings or skipping the contained headers in dependency calculations (see compiler documentation). Additionally, system include directories are searched after normal include directories regardless of the order specified."

Addresses part of #999

CMakeLists.txt

@@ -3,6 +3,7 @@
 #
 # godot-cpp cmake arguments
 # GODOT_GDEXTENSION_DIR:	Path to the directory containing GDExtension interface header and API JSON file
+# GODOT_CPP_SYSTEM_HEADERS	Mark the header files as SYSTEM. This may be useful to supress warnings in projects including this one.
 # GODOT_CUSTOM_API_FILE:	Path to a custom GDExtension API JSON file (takes precedence over `gdextension_dir`)
 # FLOAT_PRECISION:			Floating-point precision level ("single", "double")
 #
@@ -39,6 +40,7 @@ project(godot-cpp LANGUAGES CXX)
 cmake_minimum_required(VERSION 3.12)
 
 option(GENERATE_TEMPLATE_GET_NODE "Generate a template version of the Node class's get_node." ON)
+option(GODOT_CPP_SYSTEM_HEADERS "Expose headers as SYSTEM." OFF)
 
 # Default build type is Debug in the SConstruct
 if("${CMAKE_BUILD_TYPE}" STREQUAL "")
@@ -169,11 +171,19 @@ if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
 	target_compile_definitions(${PROJECT_NAME} PUBLIC TYPED_METHOD_BIND)
 endif()
 
-target_include_directories(${PROJECT_NAME} PUBLIC
+# Optionally mark headers as SYSTEM
+set(GODOT_CPP_SYSTEM_HEADERS_ATTRIBUTE "")
+if (GODOT_CPP_SYSTEM_HEADERS)
+	set(GODOT_CPP_SYSTEM_HEADERS_ATTRIBUTE SYSTEM)
+endif ()
+
+target_include_directories(${PROJECT_NAME} ${GODOT_CPP_SYSTEM_HEADERS_ATTRIBUTE} PUBLIC
 	include
 	${CMAKE_CURRENT_BINARY_DIR}/gen/include
 )
 
+unset( GODOT_CPP_SYSTEM_HEADERS_ATTRIBUTE )
+
 # Put godot headers as SYSTEM PUBLIC to exclude warnings from irrelevant headers
 target_include_directories(${PROJECT_NAME}
 	SYSTEM PUBLIC