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/http2-serverpush.c

@@ -31,6 +31,7 @@
 
 /* curl stuff */
 #include <curl/curl.h>
+#include <curl/mprintf.h>
 
 #ifndef CURLPIPE_MULTIPLEX
 #error "too old libcurl, cannot do HTTP/2 server push!"
@@ -172,7 +173,7 @@ static int server_push_callback(CURL *parent,
 
   (void)parent; /* we have no use for this */
 
-  snprintf(filename, 128, "push%u", count++);
+  curl_msnprintf(filename, 128, "push%u", count++);
 
   /* here's a new stream, save it in a new file for each new push */
   out = fopen(filename, "wb");