gh-128104: Remove Py_STRFTIME_C99_SUPPORT; require C99-compliant strftime

Modules/_datetimemodule.c

@@ -1912,9 +1912,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
         }
 #ifdef Py_NORMALIZE_CENTURY
         else if (ch == 'Y' || ch == 'G'
-#ifdef Py_STRFTIME_C99_SUPPORT
                  || ch == 'F' || ch == 'C'
-#endif
         ) {
             /* 0-pad year with century as necessary */
             PyObject *item = PySequence_GetItem(timetuple, 0);
@@ -1952,15 +1950,11 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
              * +6 to accommodate dashes, 2-digit month and day for %F. */
             char buf[SIZEOF_LONG * 5 / 2 + 2 + 6];
             Py_ssize_t n = PyOS_snprintf(buf, sizeof(buf),
-#ifdef Py_STRFTIME_C99_SUPPORT
                                       ch == 'F' ? "%04ld-%%m-%%d" :
-#endif
                                       "%04ld", year_long);
-#ifdef Py_STRFTIME_C99_SUPPORT
             if (ch == 'C') {
                 n -= 2;
             }
-#endif
             if (_PyUnicodeWriter_WriteSubstring(&writer, format, start, end) < 0) {
                 goto Error;
             }

configure.ac

@@ -6665,7 +6665,7 @@ then
   [Define if year with century should be normalized for strftime.])
 fi
 
-AC_CACHE_CHECK([whether C99-specific strftime specifiers are supported], [ac_cv_strftime_c99_support], [
+AC_CACHE_CHECK([whether C99-compatible strftime specifiers are supported], [ac_cv_strftime_c99_support], [
 AC_RUN_IFELSE([AC_LANG_SOURCE([[
 #include <time.h>
 #include <string.h>
@@ -6686,11 +6686,10 @@ int main(void)
 ]])],
 [ac_cv_strftime_c99_support=yes],
 [ac_cv_strftime_c99_support=no],
-[ac_cv_strftime_c99_support=no])])
-if test "$ac_cv_strftime_c99_support" = yes
+[ac_cv_strftime_c99_support=yes])])
+if test "$ac_cv_strftime_c99_support" = no
 then
-  AC_DEFINE([Py_STRFTIME_C99_SUPPORT], [1],
-  [Define if C99-specific strftime specifiers are supported.])
+  AC_MSG_ERROR([Python requires C99-compatible strftime specifiers])
 fi
 
 dnl check for ncursesw/ncurses and panelw/panel

pyconfig.h.in

@@ -1712,9 +1712,6 @@
 /* Define if you want to enable internal statistics gathering. */
 #undef Py_STATS
 
-/* Define if C99-specific strftime specifiers are supported. */
-#undef Py_STRFTIME_C99_SUPPORT
-
 /* The version of SunOS/Solaris as reported by `uname -r' without the dot. */
 #undef Py_SUNOS_VERSION