ci: Force build with C++11

Author: jorisv

.github/workflows/macos-linux-conda.yml

@@ -101,7 +101,8 @@ jobs:
           -DPYTHON_EXECUTABLE=$(which python3) \
           -DGENERATE_PYTHON_STUBS=ON \
           -DCMAKE_BUILD_TYPE=${{ matrix.build_type }} \
-          -DCMAKE_CXX_FLAGS=${{ matrix.cxx_options }}
+          -DCMAKE_CXX_FLAGS=${{ matrix.cxx_options }} \
+          -DCMAKE_CXX_STANDARD=11
         cmake --build . -j3
         ctest --output-on-failure
         cmake --install .