Add option to specify the desired C++ standard

Author: Faless

tools/default_flags.py

@@ -28,9 +28,9 @@ def exists(env):
 def generate(env):
     # Require C++17
     if env.get("is_msvc", False):
-        env.Append(CXXFLAGS=["/std:c++17"])
+        env.Append(CXXFLAGS=["/std:" + env["cpp_standard"]])
     else:
-        env.Append(CXXFLAGS=["-std=c++17"])
+        env.Append(CXXFLAGS=["-std=" + env["cpp_standard"]])
 
     # Disable exception handling. Godot doesn't use exceptions anywhere, and this
     # saves around 20% of binary size and very significant build time.

tools/godotcpp.py

@@ -244,6 +244,12 @@ def options(opts, env):
         )
     )
 
+    opts.Add(
+        key="cpp_standard",
+        help="The C++ standard to use. Minimum required: 'c++17'.",
+        default=env.get("cpp_standard", "c++17"),
+    )
+
     opts.Add(
         EnumVariable(
             "optimize",