cmake: separate target for examples, optimize CI, fix fallouts

- Move `docs/examples` builds under a separate target.

- Make `BUILD_EXAMPLES` default to `ON`. It means to generate the rules
  for `docs/examples` by default, but not build them. To build them,
  an explicit `make curl-examples` (or ninja, etc) command is necessary.
  This syncs behaviour with autotools, and also how both cmake and
  autotools are building tests.

- GHA: update cmake jobs to use the new way of building examples.

- GHA: move examples build step at the end of the job, after building
  and running tests. This allows to have build and test run results
  faster, and leave the seldom-changing examples build to the end.
  Building examples is the slowest build step with no practical way to
  make them fast.

- appveyor: enable building examples in two old-MSVC jobs.

- examples: fix examples to build cleanly with old MSVC versions.

- GHA/non-native: move example build log under a GHA foldable section.

- GHA/windows: move building examples into separate step for Linux cross
  jobs.

Follow-up to dfdd978f7c #13491
Closes #14906

docs/examples/cookie_interface.c

@@ -32,6 +32,7 @@
 #include <time.h>
 
 #include <curl/curl.h>
+#include <curl/mprintf.h>
 
 static void
 print_cookies(CURL *curl)
@@ -90,14 +91,11 @@ main(void)
 
     printf("-----------------------------------------------\n"
            "Setting a cookie \"PREF\" via cookie interface:\n");
-#ifdef _WIN32
-#define snprintf _snprintf
-#endif
     /* Netscape format cookie */
-    snprintf(nline, sizeof(nline), "%s\t%s\t%s\t%s\t%.0f\t%s\t%s",
-             ".example.com", "TRUE", "/", "FALSE",
-             difftime(time(NULL) + 31337, (time_t)0),
-             "PREF", "hello example, i like you!");
+    curl_msnprintf(nline, sizeof(nline), "%s\t%s\t%s\t%s\t%.0f\t%s\t%s",
+                   ".example.com", "TRUE", "/", "FALSE",
+                   difftime(time(NULL) + 31337, (time_t)0),
+                   "PREF", "hello example, i like you!");
     res = curl_easy_setopt(curl, CURLOPT_COOKIELIST, nline);
     if(res != CURLE_OK) {
       fprintf(stderr, "Curl curl_easy_setopt failed: %s\n",
@@ -110,7 +108,7 @@ main(void)
        modified, likely not what you intended. For more information refer to
        the CURLOPT_COOKIELIST documentation.
     */
-    snprintf(nline, sizeof(nline),
+    curl_msnprintf(nline, sizeof(nline),
       "Set-Cookie: OLD_PREF=3d141414bf4209321; "
       "expires=Sun, 17-Jan-2038 19:14:07 GMT; path=/; domain=.example.com");
     res = curl_easy_setopt(curl, CURLOPT_COOKIELIST, nline);