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

CMake/PickyWarnings.cmake

@@ -127,7 +127,6 @@ if(PICKY_COMPILER)
       -Wignored-qualifiers                 # clang  2.8  gcc  4.3
       -Wmissing-field-initializers         # clang  2.7  gcc  4.1
       -Wmissing-noreturn                   # clang  2.7  gcc  4.1
-      -Wno-format-nonliteral               # clang  1.0  gcc  2.96 (3.0)
       -Wno-padded                          # clang  2.9  gcc  4.1               # Not used: We cannot change public structs
       -Wno-sign-conversion                 # clang  2.9  gcc  4.3
       -Wno-switch-default                  # clang  2.7  gcc  4.1               # Not used: Annoying to fix or silence

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) {

m4/curl-compilers.m4

@@ -812,7 +812,6 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
           CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [sign-compare])
           tmp_CFLAGS="$tmp_CFLAGS -Wno-multichar"
           CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [undef])
-          tmp_CFLAGS="$tmp_CFLAGS -Wno-format-nonliteral"
           CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [endif-labels strict-prototypes])
           CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [declaration-after-statement])
           CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [cast-align])
@@ -1028,11 +1027,6 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [undef])
           fi
           #
-          dnl Only gcc 2.97 or later
-          if test "$compiler_num" -ge "297"; then
-            tmp_CFLAGS="$tmp_CFLAGS -Wno-format-nonliteral"
-          fi
-          #
           dnl Only gcc 3.0 or later
           if test "$compiler_num" -ge "300"; then
             dnl -Wunreachable-code seems totally unreliable on my gcc 3.3.2 on

src/tool_cb_prg.c

@@ -207,13 +207,13 @@ int tool_progress_cb(void *clientp,
     memset(line, '#', num);
     line[num] = '\0';
     curl_msnprintf(format, sizeof(format), "\r%%-%ds %%5.1f%%%%", barwidth);
-#ifdef __clang__
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wformat-nonliteral"
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
 #endif
     curl_mfprintf(bar->out, format, line, percent);
-#ifdef __clang__
-#pragma clang diagnostic pop
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic pop
 #endif
   }
   fflush(bar->out);

src/tool_writeout.c

@@ -579,9 +579,16 @@ static const char *outtime(const char *ptr, /* %time{ ... */
     if(!result) {
       struct tm utc;
       result = curlx_gmtime(secs, &utc);
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
+#endif
       if(curlx_dyn_len(&format) && !result &&
          strftime(output, sizeof(output), curlx_dyn_ptr(&format), &utc))
         fputs(output, stream);
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
+#endif
       curlx_dyn_free(&format);
     }
     ptr = end + 1;

tests/server/util.c

@@ -111,7 +111,15 @@ void logmsg(const char *msg, ...)
            now.tm_hour, now.tm_min, now.tm_sec, (long)tv.tv_usec);
 
   va_start(ap, msg);
+/* Suppress for builds where CURL_PRINTF() is not set */
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
+#endif
   vsnprintf(buffer, sizeof(buffer), msg, ap);
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
   va_end(ap);
 
   do {