Starting with Qt 5.7, we will require a C++11 compliant compiler to build and use Qt. 
This allows us to use many of the new features in Qt itself, and renew our codebase 
using features such as constexpr, move semantics, range-for and auto. These efforts 
are well under way and will continue throughout the next versions.

#### Qt Core
- Qt uses poll() instead of select() in it’s event loop allowing for more than 1024 file 
descriptors
- Qt now uses std::atomic for it’s atomic classes

Though it's worth noting for a future pr: 5.7 improves a few build-related
things for us:
- allows for split native tools build.
- passes flags (like -D and -B only to the host toolchain)

(I'm on mobile so those are from memory, may not be 100% accurate. I also
haven't tested that those work as intended.)

The first should allow us to split up the build into more reasonable chunks
(native_qttools and qtlibs).

The second should allow us to drop the xcb patch in favor of using more
detailed flags. It should also allow us to drop the hackish env vars we use.