Ccache depend mode

[Luboš Luňák]

 Hello,

 ccache 3.6 has introduced a new depend mode (CCACHE_DEPEND), in which ccache 
never uses preprocessing (gcc -E), which speeds things up on ccache misses.  
Previously ccache used the output from gcc -E to find out all the headers the 
source file depends on, in the depend mode it uses the output generated 
by -MD/-MMD. With https://gerrit.libreoffice.org/#/c/71629/ I'd like to 
enable this by default for LO, if supported by ccache 
(with --enable-ccache=nodepend to disable the mode).

 The advantages:

- It's faster; especially with PCH enabled:

make slideshow.build clang +ccache+icecream (ccache depend):
non-pch nodepend:
164.42user 18.05system 0:26.82elapsed 680%CPU
non-pch depend:
144.61user 13.37system 0:23.11elapsed 683%CPU
pch nodepend:
142.33user 18.04system 0:25.90elapsed 619%CPU
pch depend:
116.64user 12.10system 0:21.51elapsed 598%CPU

 The disadvantages:

- Needs recent ccache. The advantage here is that since it's so new, if there 
are problems with it, most people with not-new-enough ccache won't notice.

- There won't be any preprocessed hits. My current ccache statistics are 
141256 direct hits and 7153 preprocessed hits, so this doesn't seem to matter 
in practice.

- We use -MMD, which exludes system headers (or even our externals, since for 
those we use -isystem too). This means that ccache could give incorrect hits 
if those headers change. That may seem bad, but I think it's unlikely to 
cause problems in practice, for several reasons:
 * System headers rarely change.
 * If they change, it's generally a binary compatible change.
 * If they change in an incompatible way, the sources usually need a change 
too.
 * Since we use -MMD, our make dependencies ignore such changes as well.
This could be mitigated by switching to -MD, but that'd affect make 
performance by blowing up the number of dependencies, which I think is not 
worth it.

-- 
 Luboš Luňák
 l.lunak at collabora.com