'python3 setup.py build' fails to detect c++11 compiler

[ExecutorElassus]

I've been using a c++11-compatible gcc for about a year now, but running the command above fails with:

$ python3 setup.py build
running build
running build_ext
building 'popplerqt5' extension
x86_64-pc-linux-gnu-g++ -pthread -fPIC -I/usr/include/poppler/qt5 -I/usr/include/poppler -I/usr/include/python3.5m -I/usr/include/python3.5m -I/usr/include/qt5 -I/usr/include/qt5/QtCore -I/usr/include/qt5/QtGui -I/usr/include/qt5/QtXml -c build/temp.linux-x86_64-3.5/sippopplerqt5cmodule.cpp -o build/temp.linux-x86_64-3.5/build/temp.linux-x86_64-3.5/sippopplerqt5cmodule.o
In file included from /usr/include/qt5/QtCore/qglobal.h:83:0,
from /usr/include/qt5/QtCore/qmetatype.h:44,
from /usr/include/qt5/QtCore/QMetaType:1,
from build/temp.linux-x86_64-3.5/sipAPIpopplerqt5.h:12,
from build/temp.linux-x86_64-3.5/sippopplerqt5cmodule.cpp:7:
/usr/include/qt5/QtCore/qcompilerdetection.h:562:6: error: #error Qt requires a C++11 compiler and yours does not seem to be that.
# error Qt requires a C++11 compiler and yours does not seem to be that.

and then a long string of errors about undeclared functions.

I have python-poppler-qt5 pulled from git, current as of this message. I had also posted some more information on frescobaldi/frescobaldi#838, but which I now see was already closed by @uliska (so you might not have seen it).

My sip version is 4.19; I understand that there may be issues with ABI incompatibility with changing versions of sip. Might this be the issue? Or is something else going on here?