If we use the C language, I would prefer to only expose what does really exist, and not add "fake" functions like:

+#ifndef S_ISDOOR
+#  define S_ISDOOR(mode) 0
+#endif

So in:

+static PyMethodDef stat_methods[] = {
+    {"S_ISDIR",         stat_S_ISDIR,  METH_O, stat_S_ISDIR_doc},
+    {"S_ISCHR",         stat_S_ISCHR,  METH_O, stat_S_ISCHR_doc},
+    {"S_ISBLK",         stat_S_ISBLK,  METH_O, stat_S_ISBLK_doc},
+    {"S_ISREG",         stat_S_ISREG,  METH_O, stat_S_ISREG_doc},
+    {"S_ISFIFO",        stat_S_ISFIFO, METH_O, stat_S_ISFIFO_doc},
+    {"S_ISLNK",         stat_S_ISLNK,  METH_O, stat_S_ISLNK_doc},
+    {"S_ISSOCK",        stat_S_ISSOCK, METH_O, stat_S_ISSOCK_doc},
+    {"S_ISDOOR",        stat_S_ISDOOR, METH_O, stat_S_ISDOOR_doc},
+    {"S_ISPORT",        stat_S_ISPORT, METH_O, stat_S_ISPORT_doc},
+    {"S_ISWHT",         stat_S_ISWHT,  METH_O, stat_S_ISWHT_doc},
+    {"S_IMODE",         stat_S_IMODE,  METH_O, stat_S_IMODE_doc},
+    {"S_IFMT",          stat_S_IFMT,   METH_O, stat_S_IFMT_doc},
+    {"filemode",        stat_filemode, METH_O, stat_filemode_doc},
+    {NULL,              NULL}           /* sentinel */
+};

I expect something similar to what is done in posixmodule.c:

static PyMethodDef stat_methods[] = {
    {"S_ISDIR",         stat_S_ISDIR,  METH_O, stat_S_ISDIR_doc},
    ...
#ifdef S_ISDOOR
    {"S_ISDOOR",        stat_S_ISDOOR, METH_O, stat_S_ISDOOR_doc},
#endif
    {"filemode",        stat_filemode, METH_O, stat_filemode_doc},
    {NULL,              NULL}           /* sentinel */
};