[Bug libstdc++/119081] New: libstdc++.modules.json generated with incorrect relative path if configured with '--prefix=/' or '--prefix=' (empty)

[c at cyano dot cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119081

            Bug ID: 119081
           Summary: libstdc++.modules.json generated with incorrect
                    relative path if configured with '--prefix=/' or
                    '--prefix=' (empty)
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: c at cyano dot cn
  Target Milestone: ---

Output of `gcc -v`:

```
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/opt/gcc-git/bin/../libexec/gcc/x86_64-pc-linux-gnu/15.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure --prefix= --enable-languages=c,c++
--disable-bootstrap --disable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 15.0.1 20250301 (experimental) (GCC)
```

(Install with `sudo make DESTDIR=/opt/gcc-git install`)

Content of `/opt/gcc-git/lib64/libstdc++.modules.json`:

```json
{
  "version": 1,
  "revision": 1,
  "modules": [
    {
      "logical-name": "std",
      "source-path": "/include/c++/15.0.1/bits/std.cc",
      "is-std-library": true
    },
    {
      "logical-name": "std.compat",
      "source-path": "/include/c++/15.0.1/bits/std.compat.cc",
      "is-std-library": true
    }
  ]
}
```

The expected relative path is `../include/c++/15.0.1/bits/std.cc` and
`../include/c++/15.0.1/bits/std.compat.cc`.

It seems to be caused by `contrib/relpath.sh` not correctly handling the case:

```bash-session
$ ./contrib/relpath.sh /lib /include/c++/15.0.1/bits/std.cc
/include/c++/15.0.1/bits/std.cc

$ ./contrib/relpath.sh /usr/lib /usr/include/c++/15.0.1/bits/std.cc
../include/c++/15.0.1/bits/std.cc
```