build: Detect standard slice directories on Linux, MacOS X and FreeBSD (dev_5_0)

`ICE_HOME` and `SLICEPATH` should be unnecessary when building on Linux, MacOS X or FreeBSD using the RPMs, .debs, homebrew or ports packages, respectively.

--rebased-from #2978
--------

Testing: Build with `ICE_HOME` and `SLICEPATH` unset.  You can do e.g.

```
ant -debug clean 2>&1 | grep slicepath
```

to check what it's set to.

/cc @manics @mtbc