SCons: Don't enable `-Wenum-conversion` for GCC < 11

This warning was introduced in GCC 10 but only for C/Obj-C.
In GCC 11 it seems to have been made compatible with C++.

Also restrict -Wno-return-type to GCC 12, that regression
was fixed in GCC 13.

SConstruct

@@ -891,21 +891,24 @@ if env.msvc and not methods.using_clang(env):  # MSVC
         env.AppendUnique(LINKFLAGS=["/WX"])
 
 else:  # GCC, Clang
-    common_warnings = ["-Wenum-conversion"]
-
+    common_warnings = []
     if methods.using_gcc(env):
         common_warnings += ["-Wshadow", "-Wno-misleading-indentation"]
         if cc_version_major < 11:
             # Regression in GCC 9/10, spams so much in our variadic templates
             # that we need to outright disable it.
             common_warnings += ["-Wno-type-limits"]
-        if cc_version_major >= 12:  # False positives in our error macros, see GH-58747.
+        if cc_version_major == 12:
+            # Regression in GCC 12, false positives in our error macros, see GH-58747.
             common_warnings += ["-Wno-return-type"]
+        if cc_version_major >= 11:
+            common_warnings += ["-Wenum-conversion"]
     elif methods.using_clang(env) or methods.using_emcc(env):
         common_warnings += ["-Wshadow-field-in-constructor", "-Wshadow-uncaptured-local"]
         # We often implement `operator<` for structs of pointers as a requirement
         # for putting them in `Set` or `Map`. We don't mind about unreliable ordering.
         common_warnings += ["-Wno-ordered-compare-function-pointers"]
+        common_warnings += ["-Wenum-conversion"]
 
     # clang-cl will interpret `-Wall` as `-Weverything`, workaround with compatibility cast.
     env["WARNLEVEL"] = "-Wall" if not env.msvc else "-W3"