KWin already used C++14 constructs in a conditional way. This doesn't
make much sense today, it's better to just require C++14.
For KWin only gcc and clang are currently compilers of relevance. Gcc
supports C++14 since version 5 and defaults to C++14 since 6.1 .
Clang supports C++14 since version 3.4 .
An overview of compiler support in various distributions:
- Debian stable (stretch): gcc 6.3, clang 3.8
- Debian oldstable (jessie): 4.9, clang 3.5
- Ubuntu 17.04: gcc 6.1, clang 3.8
- Ubuntu 16.04: gcc 5.3, clang 3.8
- openSUSE Tumbleweed: gcc 7.1, clang 4.0
- openSUSE Leap 42.3: gcc ?, clang ? 
- FreeBSD: clang >= 34 in ports
- Slackware 14.2: gcc 5.3
This overview shows that every distro out there has at least one
supported compiler which can still compile KWin with this change.
 Sorry I fail to understand openSUSE's package repository.
It seems that there is gcc 7 available, but gcc package is 4.8
Test Plan: Compiles on my neon system
Subscribers: plasma-devel, kwin, KWin
Differential Revision: https://phabricator.kde.org/D6634