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/CMakeLists.txt

@@ -22,13 +22,16 @@
 #
 ###########################################################################
 
+add_custom_target(curl-examples)
+
 # Get 'check_PROGRAMS' variable
 transform_makefile_inc("Makefile.inc" "${CMAKE_CURRENT_BINARY_DIR}/Makefile.inc.cmake")
 include("${CMAKE_CURRENT_BINARY_DIR}/Makefile.inc.cmake")
 
 foreach(_target IN LISTS check_PROGRAMS)
   set(_target_name "curl-example-${_target}")
-  add_executable(${_target_name} "${_target}.c")
+  add_executable(${_target_name} EXCLUDE_FROM_ALL "${_target}.c")
+  add_dependencies(curl-examples ${_target_name})
   target_link_libraries(${_target_name} ${LIB_SELECTED} ${CURL_LIBS})
   target_compile_definitions(${_target_name} PRIVATE "CURL_NO_OLDIES")
   if(LIB_SELECTED STREQUAL LIB_STATIC AND WIN32)

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);

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");

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 */

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);
   }