As indicated in the open issue [#1320](https://github.com/alire-project/alire/issues/1320) the detection of `gmp` does not work with `alr` with the Fedora Linux OS. This PR adds the suggested missing config line to detect the Fedora `gmp-devel` library.
Tested by editing the local cache file `~/.config/alire/indexes/community/repo/index/li/libgmp/libgmp-external.toml` to add the line suggested in the open issue:
```
fedora = ["gmp-devel"]
````
The detection then works as expected, and the previously failing build of `libadalang` on Fedora 38 (x86_64) now works.
Simon Rowe
dd4f163b18