illumos/Solaris linker does not support `--exclude-libs`

To successfully compile OpenColorIO on illumos we need to disable `--exclude-libs` for linker because such option is not supported by the illumos linker. I assume the same would be true for Solaris too.

Reference: https://github.com/OpenIndiana/oi-userland/blob/e4f5cba886591d62fd365f7a941730796d2d5e46/components/library/opencolorio/patches/04-CMakeLists.txt.patch (this is for OpenColorIO 2.1.3, but 2.4.0 needs similar change too)
ld man page for illumos: https://illumos.org/man/1/ld
ld man page for Solaris: https://docs.oracle.com/cd/E88353_01/html/E37839/ld-1.html