Use ccache for CI

[Schneegans]

This may resolve #299

[coveralls]

Pull Request Test Coverage Report for Build 3657751936

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 2.934%

---

Totals
Change from base Build 3513272209:	0.0%
Covered Lines:	491
Relevant Lines:	16734

---

💛 - Coveralls

[Schneegans]

Some preliminary results. The numbers are extracted from various builds and should only be considered as rough estimates.

build type	without caching	buildcache	sccache1	ccache2	clcache3
gcc	24 min	8 min	7 min	4 min	-
gcc (pch + unity build)	12 min4	7 min	6 min	12 min	-
clang	28 min	6 min	6 min	5 min	-
clang (pch + unity build)	14 min4	7 min	6 min	5 min	-
msvc	> 60 min				?
msvc (ninja)					-
msvc (pch + unity build)	34 min	no caching5	no caching5	no caching5	28 min
msvc (pch + unity build + ninja)	31 min	fails6	32 min	23 min	-

Footnotes

1. See sccache #319 ↩

2. See ccache #320 ↩

3. The clcache numbers I cannot find anymore as the logs for those runs do not exist anymore. But I found some ancient runs which took about this time. ↩

4. Even after I switched to build type, some cache hits occurred, so the actual time could be a bit higher. ↩ ↩²

5. For some reason, I cannot get either of the tools to work with the standard Visual Studio target. With Ninja, they work though... ↩ ↩² ↩³

6. buildcache seems to have issues with precompiled headers. ↩

[Schneegans]

So I think for now, we should stick to ccache. I'll put this into this PR here and close the others.

[Schneegans]

A significant part of the time the windows build takes is for downloading boost. Maybe we should consider setting up a container for building which already includes boost. This could speed up the process by more than five minutes.