windows: use built-in _WIN32 macro to detect Windows

Windows compilers define `_WIN32` automatically. Windows SDK headers
or build env defines `WIN32`, or we have to take care of it. The
agreement seems to be that `_WIN32` is the preferred practice here.
Make the source code rely on that to detect we're building for Windows.

Public `curl.h` was using `WIN32`, `__WIN32__` and `CURL_WIN32` for
Windows detection, next to the official `_WIN32`. After this patch it
only uses `_WIN32` for this. Also, make it stop defining `CURL_WIN32`.

There is a slight chance these break compatibility with Windows
compilers that fail to define `_WIN32`. I'm not aware of any obsolete
or modern compiler affected, but in case there is one, one possible
solution is to define this macro manually.

grepping for `WIN32` remains useful to discover Windows-specific code.

Also:

- extend `checksrc` to ensure we're not using `WIN32` anymore.

- apply minor formatting here and there.

- delete unnecessary checks for `!MSDOS` when `_WIN32` is present.

Co-authored-by: Jay Satiro
Reviewed-by: Daniel Stenberg

Closes #12376

src/tool_doswin.c

@@ -23,13 +23,13 @@
  ***************************************************************************/
 #include "tool_setup.h"
 
-#if defined(MSDOS) || defined(WIN32)
+#if defined(_WIN32) || defined(MSDOS)
 
 #if defined(HAVE_LIBGEN_H) && defined(HAVE_BASENAME)
 #  include <libgen.h>
 #endif
 
-#ifdef WIN32
+#ifdef _WIN32
 #  include <stdlib.h>
 #  include <tlhelp32.h>
 #  include "tool_cfgable.h"
@@ -42,7 +42,7 @@
 #include "curlx.h"
 #include "memdebug.h" /* keep this as LAST include */
 
-#ifdef WIN32
+#ifdef _WIN32
 #  undef  PATH_MAX
 #  define PATH_MAX MAX_PATH
 #endif
@@ -55,7 +55,7 @@
 #  endif
 #endif
 
-#ifdef WIN32
+#ifdef _WIN32
 #  define _use_lfn(f) (1)   /* long file names always available */
 #elif !defined(__DJGPP__) || (__DJGPP__ < 2)  /* DJGPP 2.0 has _use_lfn() */
 #  define _use_lfn(f) (0)  /* long file names never available */
@@ -597,7 +597,7 @@ char **__crt0_glob_function(char *arg)
 
 #endif /* MSDOS && (__DJGPP__ || __GO32__) */
 
-#ifdef WIN32
+#ifdef _WIN32
 
 /*
  * Function to find CACert bundle on a Win32 platform using SearchPath.
@@ -791,6 +791,6 @@ CURLcode win32_init(void)
   return CURLE_OK;
 }
 
-#endif /* WIN32 */
+#endif /* _WIN32 */
 
-#endif /* MSDOS || WIN32 */
+#endif /* _WIN32 || MSDOS */