Bug 947256 - Report ccache hit rate at the end of the build; r=gps

This patch add a line at the end of the build like follows:
ccache (direct) hit rate: 98.7%; (preprocessed) hit rate: 0.5%; miss rate: 0.8%

--HG--
extra : rebase_source : 9c814745516d7b8b0db2056a1fa0df540a5a239a