printf: fix mingw-w64 format checks

Change mingw-w64 printf format checks in public curl headers to use
`__MINGW_PRINTF_FORMAT` instead of `gnu_printf`. This syncs the format
checker with format string macros published via `curl/system.h`. (Also
disable format checks for mingw-w64 older than 3.0.0 (2013-09-20) and
classic-mingw, which do not support this macro.)

This fixes bogus format checker `-Wformat` warnings in 3rd party code
using curl format strings with the curl printf functions, when using
mingw-w64 7.0.0 (2019-11-10) and older (with GCC, MSVCRT).

It also allows to delete two workaounds for this within curl itself:
- setting `-D__USE_MINGW_ANSI_STDIO=1` for mingw-w64 via cmake and
  configure for `docs/examples` and `tests/http/clients`.
  Ref: c730c8549b #14640

The format check macro is incompatible (depending on mingw-w64 version
and configuration) with the C99 `%z` (`size_t`) format string used
internally by curl.

To work around this problem, override the format check style in curl
public headers to use `gnu_printf`. This is compatible with `%z` in all
mingw-w64 versions and allows keeping the C99 format strings internally.

Also:
- lib/ws.c: add missing space to an error message.
- docs/examples/ftpgetinfo.c: fix to use standard printf.

Ref: #14643 (take 1)
Follow-up to 3829759bd0 #12489

Closes #14703

include/curl/curl.h

@@ -3236,7 +3236,9 @@ CURL_EXTERN CURLcode curl_easy_pause(CURL *handle, int bitmask);
 #include "options.h"
 #include "header.h"
 #include "websockets.h"
+#ifndef CURL_SKIP_INCLUDE_MPRINTF
 #include "mprintf.h"
+#endif
 
 /* the typechecker does not work in C++ (yet) */
 #if defined(__GNUC__) && defined(__GNUC_MINOR__) && \

include/curl/mprintf.h

@@ -32,13 +32,18 @@
 extern "C" {
 #endif
 
+#ifndef CURL_TEMP_PRINTF
 #if (defined(__GNUC__) || defined(__clang__) ||                         \
   defined(__IAR_SYSTEMS_ICC__)) &&                                      \
   defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) &&         \
   !defined(CURL_NO_FMT_CHECKS)
 #if defined(__MINGW32__) && !defined(__clang__)
+#if defined(__MINGW_PRINTF_FORMAT)  /* mingw-w64 3.0.0+. Needs stdio.h. */
 #define CURL_TEMP_PRINTF(fmt, arg) \
-  __attribute__((format(gnu_printf, fmt, arg)))
+  __attribute__((format(__MINGW_PRINTF_FORMAT, fmt, arg)))
+#else
+#define CURL_TEMP_PRINTF(fmt, arg)
+#endif
 #else
 #define CURL_TEMP_PRINTF(fmt, arg) \
   __attribute__((format(printf, fmt, arg)))
@@ -46,6 +51,7 @@ extern "C" {
 #else
 #define CURL_TEMP_PRINTF(fmt, arg)
 #endif
+#endif
 
 CURL_EXTERN int curl_mprintf(const char *format, ...)
   CURL_TEMP_PRINTF(1, 2);