[Nickle] Can't compile with GCC 15's -std=gnu23

[Michel Lind]

Hi Keith,

nickle up the latest git commit has a union in `gram.y` where one of the
field is `Bool bool` -- however, `bool` is now a reserved keyword

Build failure:
https://kojipkgs.fedoraproject.org//work/tasks/652/128020652/build.log

```
gram.y:87:21: error: two or more data types in declaration specifiers
   87 |     Bool            bool;
      |                     ^~~~
gram.y:87:25: warning: declaration does not declare anything
   87 |     Bool            bool;
      |                         ^
```

https://gcc.gnu.org/gcc-15/porting_to.html

(see "C23 by default")

For now I'm working around it in Fedora by switching back to -std=gnu17,
but it'd be nice to not use this workaround

https://src.fedoraproject.org/rpms/nickle/c/748bf2d8383a434195a85b4e6ce97e5687717d55?branch=rawhide

Best regards,

-- 
 _o) Michel Lind
_( ) identities: https://keyoxide.org/5dce2e7e9c3b1cffd335c1d78b229d2f7ccc04f2
     README:     https://fedoraproject.org/wiki/User:Salimma#README
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