__STDC_VERSION isn't defined when compiling C++

Currently Stg.h has the following C language version check, which ensures that the headers are being compiled as at least C99 or C++11:

#if !(__STDC_VERSION__ >= 199901L) && !(__cplusplus >= 201103L)
# error __STDC_VERSION__ does not advertise C99, C++11 or later
#endif

However, this may fail when compiling with a C++ compiler with -Wundef since __STDC_VERSION may not be defined.