Toolchain bootstrap prevents build on OSs without prebuild toolchain

commit 20215499113fc7332e70ff9bc68f90f2bf8daff6
Author: Tim Armstrong <tarmstrong@cloudera.com>
Date:   Thu Feb 11 17:31:14 2016 -0800

    IMPALA-2980: use LLVM without assertions for release build

The above commit changed the behaviour of the build scripts to always bootstrap by default. This makes sense on build machines but not for developer machines where the OS is not supported by the bootstrap script.

We should revert to the old behaviour of not bootstrapping the toolchain by default (so long as the toolchain dir is present) while allowing build jobs to opt-in to bootstrapping.