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

lib/ws.c

@@ -558,7 +558,7 @@ static ssize_t ws_enc_write_head(struct Curl_easy *data,
 
   if(enc->payload_remain > 0) {
     /* trying to write a new frame before the previous one is finished */
-    failf(data, "WS: starting new frame with %zd bytes from last one"
+    failf(data, "WS: starting new frame with %zd bytes from last one "
                 "remaining to be sent", (ssize_t)enc->payload_remain);
     *err = CURLE_SEND_ERROR;
     return -1;