Fix 24172: __STDC_VERSION__ may be defined in C++
According to the C++ ISO standard, a conformant compiler is allowed to
define this macro to any value for any reason as it is implementation
defined: https://timsong-cpp.github.io/cppwp/cpp.predefined#2.3
This mean that it cannot be assumed that it is not defined in a C++.
Change the condition to reflect that.
(cherry picked from commit 00852912edc69e652f4932fa536da60528f08ed3)