build: drop global suppression of -Wformat-nonliteral, fix fallouts

Extend two existing local suppressions to GCC, and add another
GCC-specific one as a replacement.

Before this patch suppressing this warning was odd with clang, because
after this option, `-Wformat=2` is used, which re-enables it.

Also:
- mprintf: minimize scope of a warning suppression.
- tests/server: suppress this warning for a system `vsnprintf()` call
  where it could trigger in C89 builds or with
  `CFLAGS=-DCURL_NO_FMT_CHECKS` set. Seen with Apple clang 17:
  ```
  curl/tests/server/util.c:114:37: warning: format string is not a string literal [-Wformat-nonliteral]
    114 |   vsnprintf(buffer, sizeof(buffer), msg, ap);
        |                                     ^~~
  /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/secure/_stdio.h:124:69: note: expanded from macro 'vsnprintf'
    124 | #define vsnprintf(str, len, ...) __vsnprintf_chk_func (str, len, 0, __VA_ARGS__)
        |                                                                     ^~~~~~~~~~~
  /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/secure/_stdio.h:81:65: note: expanded from macro '__vsnprintf_chk_func'
     81 |         __builtin___vsnprintf_chk (str, len, flag, __darwin_obsz(str), format, ap)
        |                                                                        ^~~~~~
  ```

Ref: #20363

Closes #20366

lib/mprintf.c

@@ -673,16 +673,19 @@ static bool out_double(void *userp,
 
   *fptr = 0; /* and a final null-termination */
 
-#ifdef __clang__
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wformat-nonliteral"
-#endif
   /* NOTE NOTE NOTE!! Not all sprintf implementations return number of
      output characters */
 #ifdef HAVE_SNPRINTF
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
+#endif
   /* !checksrc! disable LONGLINE */
   /* NOLINTNEXTLINE(clang-analyzer-security.insecureAPI.DeprecatedOrUnsafeBufferHandling) */
   (snprintf)(work, BUFFSIZE, formatbuf, dnum);
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
 #ifdef _WIN32
   /* Old versions of the Windows CRT do not terminate the snprintf output
      buffer if it reaches the max size so we do that here. */
@@ -691,9 +694,6 @@ static bool out_double(void *userp,
 #else
   /* float and double outputs do not work without snprintf support */
   work[0] = 0;
-#endif
-#ifdef __clang__
-#pragma clang diagnostic pop
 #endif
   DEBUGASSERT(strlen(work) < BUFFSIZE);
   while(*work) {