[Development] Raising the minimum Core Language to C++14 for Linux/XCB

[Thiago Macieira]

On Mac, we kinda already require that. I'm asking to raise the minimum for the 
regular Linux builds to C++14. Specifically, I'm asking for the "auto" 
functions without trailing return type and relaxed constexpr.

Currently, our minimum supported GCC version is 4.7 (on QNX only) and 4.8 
elsewhere. That would raise the minimum on Linux to GCC 5, as per:
	https://gcc.gnu.org/projects/cxx-status.html#cxx14

That's a 3-year-old compiler, four releases out of date, present in the main, 
binary Linux distros since:
	Ubuntu 15.10		[2015]
	Fedora 22		[2015]
	Debian 9 (Stretch)	[2017]
	openSUSE Leap 15 [2018]

Note that this does not apply to QNX or Android, so C++14 features would not 
be allowed in cross-platform code.

But we'd be able to use it in the XCB plugin. [Does QNX build that?]

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center