Restore use of ccache for CI.

4 jobs for <span class="ref-name">feature/restore-use-of-ccache-for-ci</span> in 22 minutes and 9 seconds (queued for 30 seconds)
Status Job ID Name Coverage
  Test
passed #7714
docker-debian9
job-debian9-ninja-clang

00:02:50

passed #7713
docker-debian9
job-debian9-ninja-gcc

00:10:13

passed #7715
macosx
job-macosx-ninja

00:13:40

passed #7716
windows
job-windows-vs2015

00:11:56