When devtoolset-3 is used (via enable_devtoolset.sh on el6), it's quite likely that ccache will go unused for the build. Certainly for build-thirdparty.sh, and likely for the main Kudu build too (unless you go out of your way to set CC/CXX=ccache when invoking cmake).
We should be able to fix this in enable_devtoolset.sh, at least in the common case where symlinks to ccache named after the compiler are on the PATH. We could ensure that, following the call to 'scl enable devtoolset-3 <cmd>', ccache symlinks are placed at the head of the PATH, before /opt/rh/devtoolset-3/, and only then is <cmd> actually invoked. This should cause ccache to be used, and it'll chain to the devtoolset-3 compiler because /opt/rh/devtoolset-3/ is ahead of /usr/bin on the PATH. We may need an intermediate script to do this.