Bug#906708: hepmc: FTBFS on i386 / arm64 / ppc64el / s390x

[Lumin]

control: tags -1 +patch +confirmed

> Fix attached.

> -  double eps = 1.e-15; // allowed differnce between doubles
> +  double eps = 4.e-15; // allowed difference between doubles

No no no, please DON'T do this!

This precision issue is triggered by a GCC optimization bug,
and simply bumping floating point tolerance for scientific software
especially physics software is bad....

The fix of this RC is to simply change the default compiler to Clang.
Or disable -O2 optimization with -O0

the real patch:

debian/rules:
export CC=clang
export CXX=clang++