Can't deal with "Qt requires a C++11 compiler and yours does not seem to be that."

I need to use both Qt and Boost in my project. The versions are:
Qt:qt 5.9.1, qmake.
Boost: 1_65_1
Compiler: MSVC17 64bit.

I'm certain I did compile boost in MSVC17, and I chose it as a main compiler for my Qt project. Though, it doesn't work. It throws the error I put in the title.
Here are a few things I tried.

CONFIG += c++11
No effect
QMAKE_CXXFLAGS += -std=c++11
ignoring unknown option '-std=c++11'. I'm not sure if it should work with the MS compiler

I saw a few questions with similar problems, but it didn't help.