Build failure with GCC 15 (defaults to -std=gnu23)

Hi,

postgres-17.1 fails to build with upcoming GCC 15 which defaults to
-std=gnu23 as follows:
```
In file included from ../../src/include/postgres_fe.h:25,
from checksum_helper.c:17:
../../src/include/c.h:456:23: error: two or more data types in declaration specifiers
456 | typedef unsigned char bool;
| ^~~~
../../src/include/c.h:456:1: warning: useless type name in empty declaration
456 | typedef unsigned char bool;
| ^~~~~~~
```

src/include/c.h does attempt to check for whether bool is already defined but
the check doesn't work.

There may be more issues afterwards but I haven't tried papering over
the above issue. It should be possible to reproduce w/
CFLAGS="... -std=gnu23" or CFLAGS="... -std=c23" on older GCC or Clang.

thanks,
sam