bug 1481614 - detect icecream usage in build telemetry. r=chmanchester,glandium

This patch adds detection for when icecream is in use to build telemetry.
icecream is commonly enabled in two ways: by either setting CC/CXX to point
to icecream's cc/c++ symlinks, or by setting adding
mk_add_options 'export CCACHE_PREFIX=icecc' to a mozconfig when also using
ccache. For the former, this patch adds a simple configure check to see
if CXX is a symlink to a file named 'icecc'. For the latter this patch adds
CCACHE_PREFIX as a configure subst to capture the value.

We don't currently have a facility for writing telemetry tests that depend on
configure values. Local manual testing shows that it does work as expected.

Differential Revision: https://phabricator.services.mozilla.com/D18138

--HG--
extra : moz-landing-system : lando