[Issue 21177] New: printf check does not recognize GNU extensions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21177

          Issue ID: 21177
           Summary: printf check does not recognize GNU extensions
           Product: D
           Version: D2
          Hardware: All
                OS: Linux
            Status: NEW
          Keywords: rejects-valid
          Severity: regression
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

I have some platform-specific server code (it only runs on Ubuntu / Glibc).
In this code, the `%m` format is used with fscanf:
```
                /*
                 * Special fscanf format tokens:
                 *   - The leading ' ' skips leading white space.
                 *   - %n stores the current position in the input string in
the
                 *     argument so that
                 *     channel_name.length = name_end - name_start.
                 *   - %m matches a string, stores it in a buffer allocated by
                 *     malloc and stores a pointer to that buffer in the
                 *     argument.
                 *   - [_0-9a-zA-Z-] makes %m match only strings that consist
of
                 *     the characters '_', '0'-'9', 'a'-'z', 'A'-'Z' or '-',
                 *     which ensures the string is a valid dls channel name.
                 */
                n = fscanf(this.stream,
                           " %n%m[_0-9a-zA-Z-]%n %lu %lu".ptr,
                           &name_start, &channel_name, &name_end,
                           &bucket_start, &bucket_offset)
```
Source:
https://github.com/sociomantic-tsunami/dlsnode/blob/26a8037827a653e7827ead19d693357e2c808723/src/dlsnode/storage/checkpoint/CheckpointFile.d#L203-L219

While trying to update to the latest DMD, I get a deprecation message.
>From a quick google search, I could find the following extension:
- `%m` for printf =>  Print output of strerror(errno) (no argument)
- `%a` for scanf
- `%m` for scanf

Note that a "smart" fix would only warn about those when not using
`CRuntime_Glibc`. Or just accept that GNU extensions are widespread and let
those slide.

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