check for standalone libiconv if not built in libc

On FreeBSD libiconv need to be manually linked because there it's not built in the libc. However the `CHECK_FUNCTION_EXISTS` check succeeds.

The file README.PLATFORMS mentions this, but the workaround doesn't work since we're using cmake.
So the intention is that cmake checks dynamically:
If we find a iconv library, we can assume to use it otherwise we just check if the function exists which then indicates that the libc built-in version is being used.