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

`ICE_HOME` and `SLICEPATH` should be unnecessary when building on Linux, MacOS X or FreeBSD using the RPMs, .debs, homebrew or ports packages, respectively.

--------

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 
Yes.  We could error out here but IIRC we don't since not all targets require it to be valid.