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/sendrecv.c

@@ -51,6 +51,9 @@ static int wait_on_socket(curl_socket_t sockfd, int for_recv, long timeout_ms)
 #if defined(__GNUC__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wsign-conversion"
+#elif defined(_MSC_VER)
+#pragma warning(push)
+#pragma warning(disable:4127)  /* conditional expression is constant */
 #endif
   FD_SET(sockfd, &errfd); /* always check for error */
 
@@ -62,6 +65,8 @@ static int wait_on_socket(curl_socket_t sockfd, int for_recv, long timeout_ms)
   }
 #if defined(__GNUC__)
 #pragma GCC diagnostic pop
+#elif defined(_MSC_VER)
+#pragma warning(pop)
 #endif
 
   /* select() returns the number of signalled sockets or -1 */