Skip unsupported linker options on illumos.

Skip unsupported linker options on illumos.

Building on illumos currently fails with:

```
ld: fatal: unrecognized option '--gc-sections'
```

This happens because `--gc-sections` isn't supported on illumos `ld`.
This patch updates CMakeLists.txt to skip unsupported linker options on
illumos. The flags used on other operating systems are optimizations
that don't affect correctness, so this change is safe.