choosenim fails to detect clang compiler on linux
When installed on system with only `clang` compiler present choosenim fails to detect presence of C compiler.

When installed via `curl https://nim-lang.org/choosenim/init.sh -sSf | sh` with `clang` in `/usr/bin/clang` I get following error:

```bash
     Error: No C compiler found. Nim compiler requires a C compiler.
        ... Install clang or gcc using your favourite package manager.
```

even though `whereis clang`  can find the compiler binary

```
> whereis clang
clang: /usr/bin/clang /usr/lib/clang /usr/include/clang
```

Installing `gcc` compiler resolves the problem and nim is correctly installed.