patch 8.1.0941: macros for MS-Windows are inconsistent

Problem:    Macros for MS-Windows are inconsistent, using "32", "3264 and
            others.
Solution:   Use MSWIN for all MS-Windows builds.  Use FEAT_GUI_MSWIN for the
            GUI build. (Hirohito Higashi, closes #3932)
diff --git a/src/winclip.c b/src/winclip.c
index 0285dc5..2d4d97e 100644
--- a/src/winclip.c
+++ b/src/winclip.c
@@ -22,7 +22,7 @@
  * posix environment.
  */
 #ifdef FEAT_CYGWIN_WIN32_CLIPBOARD
-# define WIN3264
+# define MSWIN
 # define WIN32_LEAN_AND_MEAN
 # include <windows.h>
 # include "winclip.pro"
@@ -299,7 +299,7 @@
     VimClipType_t	metadata = { -1, -1, -1, -1 };
     HGLOBAL		hMem = NULL;
     char_u		*str = NULL;
-#if defined(WIN3264)
+#if defined(MSWIN)
     char_u		*to_free = NULL;
 #endif
     HGLOBAL		rawh = NULL;
@@ -361,7 +361,7 @@
     }
     if (str == NULL)
     {
-#if defined(WIN3264)
+#if defined(MSWIN)
 	/* Try to get the clipboard in Unicode if it's not an empty string. */
 	if (IsClipboardFormatAvailable(CF_UNICODETEXT) && metadata.ucslen != 0)
 	{
@@ -417,7 +417,7 @@
 			    break;
 		}
 
-#if defined(WIN3264)
+#if defined(MSWIN)
 		/* The text is in the active codepage.  Convert to
 		 * 'encoding', going through UTF-16. */
 		acp_to_enc(str, str_size, &to_free, &maxlen);
@@ -454,7 +454,7 @@
     if (rawh != NULL)
 	GlobalUnlock(rawh);
     CloseClipboard();
-#if defined(WIN3264)
+#if defined(MSWIN)
     vim_free(to_free);
 #endif
 }
@@ -471,7 +471,7 @@
     HGLOBAL		hMemRaw = NULL;
     HGLOBAL		hMem = NULL;
     HGLOBAL		hMemVim = NULL;
-# if defined(WIN3264)
+# if defined(MSWIN)
     HGLOBAL		hMemW = NULL;
 # endif
 
@@ -508,7 +508,7 @@
 	    metadata.rawlen = 0;
     }
 
-# if defined(WIN3264)
+# if defined(MSWIN)
     {
 	WCHAR		*out;
 	int		len = metadata.txtlen;
@@ -590,7 +590,7 @@
 	{
 	    SetClipboardData(cbd->format, hMemVim);
 	    hMemVim = 0;
-# if defined(WIN3264)
+# if defined(MSWIN)
 	    if (hMemW != NULL)
 	    {
 		if (SetClipboardData(CF_UNICODETEXT, hMemW) != NULL)
@@ -611,7 +611,7 @@
 	GlobalFree(hMemRaw);
     if (hMem)
 	GlobalFree(hMem);
-# if defined(WIN3264)
+# if defined(MSWIN)
     if (hMemW)
 	GlobalFree(hMemW);
 # endif
@@ -746,7 +746,7 @@
     return enc_str;
 }
 
-#if defined(WIN3264) || defined(PROTO)
+#if defined(MSWIN) || defined(PROTO)
 /*
  * Convert from the active codepage to 'encoding'.
  * Input is "str[str_size]".