C++: export /usr/lib/ccache to the PATH
C++: export /usr/lib/ccache to the PATH

C builds already have this feature but this was missing for C++.

Signed-off-by: David Wagner <60228a510f9a76940848fa2d29321fc4edfc2119@intel.com>

andyli/travis-build,kevmoo/travis-build,andyli/travis-build,kevmoo/travis-build,jhass/travis-build,jhass/travis-build,akoeplinger/travis-build,akoeplinger/travis-build,craigcitro/travis-build,craigcitro/travis-build,JuliaCI/travis-build,JuliaCI/travis-build