Ccache depend mode

[Michael Stahl]

On 07.05.19 11:32, Luboš Luňák wrote:

> - We use -MMD, which exludes ... headers (or even our externals, since for
> those we use -isystem too).

no, -isystem can't be used with bundled externals because it breaks 
incremental builds.... with -I we get deps on the headers [to be 
rewritten into deps on the UnpackedTarball target by concat-deps], and 
since we don't copy the headers out of the UnpackedTarball dir 
incremental builds tend to work fine, with oboslete .so.N files 
littering instdir as the only problem typically.

>   * 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.

agree; typically system header files only change in incompatible ways on 
distro-upgrades so you know to "make clean" twice a year.