gcc-6 + osx build failure

[soonho-tri]

Reported by @scungao (using 10.11):

[ 64%] Linking CXX executable dOp
Undefined symbols for architecture x86_64:
 "capd::dynset::C0DoubletonSet<capd::vectalg::Matrix<capd::intervals::Interval<double, capd::rounding::DoubleRounding>, 0u, 0u>, capd::dynset::FactorReorganization<capd::dynset::FullQRWithPivoting<capd::dynset::DefaultPolicy> > >::show[abi:cxx11]() const", referenced from:
     vtable for capd::dynset::C0DoubletonSet<capd::vectalg::Matrix<capd::intervals::Interval<double, capd::rounding::DoubleRounding>, 0u, 0u>, capd::dynset::FactorReorganization<capd::dynset::FullQRWithPivoting<capd::dynset::DefaultPolicy> > > in libdreal.a(contractor_capd4.cpp.o)
 "capd::geomset::DoubletonSet<capd::vectalg::Matrix<capd::intervals::Interval<double, capd::rounding::DoubleRounding>, 0u, 0u> >::toString[abi:cxx11]() const", referenced from:
     vtable for capd::geomset::DoubletonSet<capd::vectalg::Matrix<capd::intervals::Interval<double, capd::rounding::DoubleRounding>, 0u, 0u> > in libdreal.a(contractor_capd4.cpp.o)
     vtable for capd::dynset::C0DoubletonSet<capd::vectalg::Matrix<capd::intervals::Interval<double, capd::rounding::DoubleRounding>, 0u, 0u>, capd::dynset::FactorReorganization<capd::dynset::FullQRWithPivoting<capd::dynset::DefaultPolicy> > > in libdreal.a(contractor_capd4.cpp.o)
 "capd::geomset::AffineSet<capd::vectalg::Matrix<capd::intervals::Interval<double, capd::rounding::DoubleRounding>, 0u, 0u> >::toString[abi:cxx11]() const", referenced from:
     vtable for capd::geomset::AffineSet<capd::vectalg::Matrix<capd::intervals::Interval<double, capd::rounding::DoubleRounding>, 0u, 0u> > in libdreal.a(contractor_capd4.cpp.o)
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
make[2]: *** [dOp] Error 1
make[1]: *** [CMakeFiles/dOp-bin.dir/all] Error 2
make: *** [all] Error 2

[AlyssaLytle]

I had the same issue. I ended up just installing gcc5 and using that.

brew install gcc5