Backport of core PR10779 and PR11264
https://github.com/bitcoin/bitcoin/pull/10779/files
https://github.com/bitcoin/bitcoin/pull/11264/files
As OpenBSD is no longer supported the doc update is not applicable.
The versions used are set according to our depends system.
The minimum versions:
- Clang and GCC: set to the first version with complete support for
C++14.
- libevent: minimum 2.0.22 as 2.0.21 has a memory leak and would cause
asan to fail.
- Qt: 5.5.1 is Ubuntu Xenial version, and matches core's master
minimum version. >= 5.4 is required to allow for QTimer::singleShot to
build.