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/simplessl.c

@@ -83,6 +83,10 @@ int main(void)
     curl_easy_setopt(curl, CURLOPT_URL, "HTTPS://your.favourite.ssl.site");
     curl_easy_setopt(curl, CURLOPT_HEADERDATA, headerfile);
 
+#ifdef _MSC_VER
+#pragma warning(push)
+#pragma warning(disable:4127)  /* conditional expression is constant */
+#endif
     do { /* dummy loop, just to break out from */
       if(pEngine) {
         /* use crypto engine */
@@ -133,6 +137,9 @@ int main(void)
 
       /* we are done... */
     } while(0);
+#ifdef _MSC_VER
+#pragma warning(pop)
+#endif
     /* always cleanup */
     curl_easy_cleanup(curl);
   }