bpo-46816: Remove declarations for non-__STDC__ compilers (GH-31466)

After Python switched to C11, there is a guarantee that every compiler
conforms to ISO C so checks of __STDC__ have no sense.

Include/internal/pycore_pymath.h

@@ -8,15 +8,6 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
-// Extra declarations
-#if !defined(_MSC_VER) && !defined(__STDC__)
-extern double fmod (double, double);
-extern double frexp (double, int *);
-extern double ldexp (double, int);
-extern double modf (double, double *);
-extern double pow(double, double);
-#endif  // !defined(_MSC_VER) && !defined(__STDC__)
-
 
 /* _Py_ADJUST_ERANGE1(x)
  * _Py_ADJUST_ERANGE2(x, y)

Python/errors.c

@@ -10,12 +10,6 @@
 #include "pycore_sysmodule.h"     // _PySys_Audit()
 #include "pycore_traceback.h"     // _PyTraceBack_FromFrame()
 
-#ifndef __STDC__
-#ifndef MS_WINDOWS
-extern char *strerror(int);
-#endif
-#endif
-
 #include <ctype.h>
 #ifdef MS_WINDOWS
 #  include <windows.h>