fix: Build with CXX11 and ROOT7 OFF

Axel has pointed out that gcc v4.8 doesn't support C++14 code and
so CXX11 should be used. As ROOT7 requires CXX14 then additionally
turn ROOT7 off.

c.f. https://root-forum.cern.ch/t/root-v6-24-00-build-failure-in-centos-7-docker-image-on-auto-return-without-trailing-return-type/45238/6