PG_CFLAGS rpath Passthrough Issue

Hello Hackers,

I'm trying to compile an extension with PG_CFLAGS1[1]:

```sh
make PG_CFLAGS='-Wl,-rpath,$ORIGIN'
```

This works, but for some reason the rpath value is truncated:

```console
# chrpath -l src/semver.so
src/semver.so: RUNPATH=RIGIN
```

Do I need to do something different to include the missing characters `$O`? Or is there an issue with the quoting of
thesevariables in PGXS? 

Thanks,

David

[1]: https://github.com/postgres/postgres/blob/c7fc880/src/makefiles/pgxs.mk#L58