build: fix compiling with GCC 4.x versions

- silence false positive picky warnings.
- avoid "possible noreturn" warnings for standalone tests and examples.
- fix to compile without `#pragma GCC diagnostic push` support.
- fix "#pragma GCC diagnostic not allowed inside functions".

Prerequisite for #15975 that needs GCC 4.4 for the latest pre-built
CeGCC/mingw32ce toolchain for Windows CE.

Cherry-picked from #15975
Closes #16062

docs/examples/cacertinmem.c

@@ -31,6 +31,10 @@
 #include <curl/curl.h>
 #include <stdio.h>
 
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic ignored "-Woverlength-strings"
+#endif
+
 static size_t writefunction(void *ptr, size_t size, size_t nmemb, void *stream)
 {
   fwrite(ptr, size, nmemb, (FILE *)stream);
@@ -41,11 +45,6 @@ static CURLcode sslctx_function(CURL *curl, void *sslctx, void *parm)
 {
   CURLcode rv = CURLE_ABORTED_BY_CALLBACK;
 
-#if defined(__GNUC__) || defined(__clang__)
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Woverlength-strings"
-#endif
-
   /** This example uses two (fake) certificates **/
   static const char mypem[] =
     "-----BEGIN CERTIFICATE-----\n"
@@ -90,10 +89,6 @@ static CURLcode sslctx_function(CURL *curl, void *sslctx, void *parm)
     "Z05phkOTOPu220+DkdRgfks+KzgHVZhepA==\n"
     "-----END CERTIFICATE-----\n";
 
-#if defined(__GNUC__) || defined(__clang__)
-#pragma GCC diagnostic pop
-#endif
-
   BIO *cbio = BIO_new_mem_buf(mypem, sizeof(mypem));
   X509_STORE  *cts = SSL_CTX_get_cert_store((SSL_CTX *)sslctx);
   int i;

docs/examples/sendrecv.c

@@ -30,6 +30,19 @@
 #include <string.h>
 #include <curl/curl.h>
 
+/* Avoid warning in FD_SET() with pre-2020 Cygwin/MSYS releases:
+ * warning: conversion to 'long unsigned int' from 'curl_socket_t' {aka 'int'}
+ * may change the sign of the result [-Wsign-conversion]
+ */
+#ifdef __GNUC__
+#pragma GCC diagnostic ignored "-Wsign-conversion"
+#ifdef __DJGPP__
+#pragma GCC diagnostic ignored "-Warith-conversion"
+#endif
+#elif defined(_MSC_VER)
+#pragma warning(disable:4127)  /* conditional expression is constant */
+#endif
+
 /* Auxiliary function that waits on the socket. */
 static int wait_on_socket(curl_socket_t sockfd, int for_recv, long timeout_ms)
 {
@@ -49,20 +62,6 @@ static int wait_on_socket(curl_socket_t sockfd, int for_recv, long timeout_ms)
   FD_ZERO(&outfd);
   FD_ZERO(&errfd);
 
-/* Avoid this warning with pre-2020 Cygwin/MSYS releases:
- * warning: conversion to 'long unsigned int' from 'curl_socket_t' {aka 'int'}
- * may change the sign of the result [-Wsign-conversion]
- */
-#if defined(__GNUC__)
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wsign-conversion"
-#if defined(__DJGPP__)
-#pragma GCC diagnostic ignored "-Warith-conversion"
-#endif
-#elif defined(_MSC_VER)
-#pragma warning(push)
-#pragma warning(disable:4127)  /* conditional expression is constant */
-#endif
   FD_SET(sockfd, &errfd); /* always check for error */
 
   if(for_recv) {
@@ -71,11 +70,6 @@ static int wait_on_socket(curl_socket_t sockfd, int for_recv, long timeout_ms)
   else {
     FD_SET(sockfd, &outfd);
   }
-#if defined(__GNUC__)
-#pragma GCC diagnostic pop
-#elif defined(_MSC_VER)
-#pragma warning(pop)
-#endif
 
   /* select() returns the number of signalled sockets or -1 */
   res = select((int)sockfd + 1, &infd, &outfd, &errfd, &tv);

docs/examples/usercertinmem.c

@@ -41,6 +41,10 @@
 #include <curl/curl.h>
 #include <stdio.h>
 
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic ignored "-Woverlength-strings"
+#endif
+
 static size_t writefunction(void *ptr, size_t size, size_t nmemb, void *stream)
 {
   fwrite(ptr, size, nmemb, stream);
@@ -55,11 +59,6 @@ static CURLcode sslctx_function(CURL *curl, void *sslctx, void *parm)
   RSA *rsa = NULL;
   int ret;
 
-#if defined(__GNUC__) || defined(__clang__)
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Woverlength-strings"
-#endif
-
   const char *mypem = /* www.cacert.org */
     "-----BEGIN CERTIFICATE-----\n"\
     "MIIHPTCCBSWgAwIBAgIBADANBgkqhkiG9w0BAQQFADB5MRAwDgYDVQQKEwdSb290\n"\
@@ -124,10 +123,6 @@ static CURLcode sslctx_function(CURL *curl, void *sslctx, void *parm)
     "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX\n"\
     "-----END RSA PRIVATE KEY-----\n";
 
-#if defined(__GNUC__) || defined(__clang__)
-#pragma GCC diagnostic pop
-#endif
-
   (void)curl; /* avoid warnings */
   (void)parm; /* avoid warnings */