cext: define S_IFLNK on Python 2.7 and Windows

Before this change, building on Python 2.7 on Windows fails due to
S_IFLNK being undefined. This regression was introduced by
a32a96079e2d / D11518.

It worked on Python 3 because its pyport.h (which is included via
Python.h) contains effectively the same code as added by this
changeset.

Differential Revision: https://phab.mercurial-scm.org/D11763

mercurial/cext/parsers.c

@@ -25,6 +25,12 b''
 #define PyInt_FromLong PyLong_FromLong
 #define PyInt_FromSsize_t PyLong_FromSsize_t
 #define PyInt_AsLong PyLong_AsLong
+#else
+/* Windows on Python 2.7 doesn't define S_IFLNK. Python 3+ defines via
+ * pyport.h. */
+#ifndef S_IFLNK
+#define S_IFLNK 0120000
+#endif
 #endif
 
 static const char *const versionerrortext = "Python minor version mismatch";