possible build issues not covered by CI

[scravy]

We have encountered numerous build issues that are not covered by our CI, which is why I recommend to only approve pull requests that have at least one ACK.

Possible build issues, not so esoteric ones, which we have encountered, some of them numerous times:

• unit tests pass when run using make check, but not when run individually (happened often, but was always followed up on promptly, so no ticket issued)
• unit tests do not pass when run in parallel (current issue, blocker for merging 0.17)
  • we will detect it once we have https://github.com/dtr-org/unit-e/pull/865 in
• build works in travis but not on more recent versions of GCC/Clang (recent #include <functional> failure)
• bench broken (https://github.com/dtr-org/unit-e/issues/722)
  • we will detect it once we have https://github.com/bitcoin/bitcoin/pull/13811 in
• additional compiler warnings introduced
  • The only way to check is to check the diff between a previous build and the current build or to not have any compiler warnings at all and fail the build (-Werror) in case we do get any (https://github.com/dtr-org/unit-e/issues/75)
• regtest works, but when started up for mainnet/testnet node crashes (we had that)
• gitian build fails
  • bitcoin has labels "needs gitian build" and a bot which provides a gitian build when attached so reviewers can convince themselves release-pipeline is not broken