Refined condition for PYBIND11_WORKAROUND_INCORRECT_OLD_GCC_UNUSED_BUT_SET_PARAMETER.

Author: rwgk

include/pybind11/detail/common.h

@@ -943,7 +943,7 @@ inline void silence_unused_warnings(Args &&...) {}
 #if defined(__GNUG__) && !defined(__clang__) && !defined(__INTEL_COMPILER)
 #    define PYBIND11_WORKAROUND_INCORRECT_ALL_GCC_UNUSED_BUT_SET_PARAMETER(...)                   \
         detail::silence_unused_warnings(__VA_ARGS__)
-#    if defined(__GNUC__) && __GNUC__ <= 2
+#    if defined(__GNUC__) && __GNUC__ <= 5 // 4 is certain, 5 unknown, 6 is certain
 #        define PYBIND11_WORKAROUND_INCORRECT_OLD_GCC_UNUSED_BUT_SET_PARAMETER(...)               \
             detail::silence_unused_warnings(__VA_ARGS__)
 #    else