cmake: add ccache support

On my not-so-fast NUC it improves build time:

clean build without ccache (and first build with ccache):
real	6m15.062s
user	19m45.633s
sys	1m48.421s

second clean build with ccache:
real	0m48.836s
user	1m48.793s
sys	0m35.407s

It's not needed for CI, as builds are done in containers and do not share
ccache anyway