patch 9.1.0502: MS-Windows: too much legacy code

Problem:  MS-Windows: too much legacy code
Solution: Clean up old code
          (Ken Takata)

* Remove very old codes for Cygwin version of GCC.
  Nowadays Cygwin GCC cannot be used for building Win32 Vim.
  (The `-mno-cygwin` option was removed in Cygwin GCC4.)
* Remove old codes for old versions of MinGW.
  Remove `__MINGW32__` as much as possible.
* Adjust makefile.

closes: #15044

Signed-off-by: K.Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/version.c b/src/version.c
index 2d65eb2..3afaefa 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    502,
+/**/
     501,
 /**/
     500,