Why is _LARGEFILE_SOURCE defined in stdio.h when compiling with g++ but not gcc?

If the following code is compiled with gcc lfs.c -o lfs, it prints nothing. However, if it is compiled with g++ lfs.c -o lfs, it prints "_LARGEFILE_SOURCE defined by stdio.h!".

#ifdef _LARGEFILE_SOURCE
int largefile_defined_at_start = 1;
#else
int largefile_defined_at_start = 0;
#endif

// This defines _LARGEFILE_SOURCE, but only in C++!
#include <stdio.h>

int main(void) {
#ifdef _LARGEFILE_SOURCE
  if (!largefile_defined_at_start)
    printf("_LARGEFILE_SOURCE defined by stdio.h!");
#endif
  return 0;
}

In either case, _LARGEFILE_SOURCE is not defined by the compiler:

gcc -dM -E - < /dev/null |grep _LARGEFILE_SOURCE |wc -l
0
g++ -dM -E - < /dev/null |grep _LARGEFILE_SOURCE |wc -l
0

Why is stdio.h defining _LARGEFILE_SOURCE when GCC is invoked via the g++ frontend?