Enable pipefail, and stuff both tar and grep in subshells, to work around grep modifying the return code of tar. This should let us filter the excessive warning messages when GNU tar encounters those "SCHILY.dev" fields of star, while still allowing the shell to terminate the process when tar itself fails.
Unless I'm mistaken, the '|| echo ".dirty"' seems to make it so that ".dirty" is always attached, because it modifies the exit code. At least echoing XOREOS_REVDIRT right after the execute_process suggests that. With the '|| echo ".dirty"', it's always 128 for me, no matter if the tree is dirty or not. Without, it evaluates to either 0 or 1, correctly showing the dirty-state.
Includes fixes after berenm's review comments (see cc9cii@4a43611)
We're blocked by travis-ci/apt-source-whitelist#45, which would add a pure Ubuntu source for the SDL2 packages.
Let's try using the debian-sid APT source...
Previously, we used ZLIB_VER_MAJOR, ZLIB_VER_MINOR, ZLIB_VER_REVISION and ZLIB_VER_SUBREVISION; but ZLIB_VER_SUBREVISION wasn't added until 1.2.4 (and we claim to need only 188.8.131.52).
If set to false, don't save a changed configuration. This basically write-protects the config file.