Most compilers out in the wild still don't support the flag, so we need
to compare the version number anyway. This also makes it ready for
whenever compilers start supporting -std=c++14, something we should fix
for C++11 too.
It overrides the CXX11 variable for two reasons:
1) we reuse the mechanics in c++11.prf
2) we avoid c++11.prf overriding the flag if qmake decides to process
it later (CONFIG += c++14 is additive)
Change-Id: I79b6523fd9017483f2474634d1c09f2fd5ea039d
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
