[Rivet] Rivet build fails with gcc71

[Dmitri Konstantinov]

Dear Rivet developers,

In CERN we have started to build LCG software stack with gcc 7.1 (I) and 
there is a problem with Rivet.

http://cdash.cern.ch/upload/1b4440604d07f796afec84d4ceb28e3c1db321e2/rivet-2.5.3-build.log

Rivet configure ( m4/ax_cxx_compile_stdcxx.m4  macro)  doesn't set C++11 
flag thinking that gcc 7 supports C++11 by default (II) but

apparently not all features as it fails to compile.

Could you please take a look?

We see that it can be bypassed by passing c++11 with CXXFLAGS .

Cheers,

    Dima for the GENSER


(I)  source /cvmfs/sft.cern.ch/lcg/contrib/gcc/7/x86_64-slc6/setup.sh

(II) https://rivet.hepforge.org/trac/browser/m4/ax_cxx_compile_stdcxx.m4#L62