using gcc 7.2.0 fails to build some tests

[fnrizzi]

for example:

pressio/tests/functional_small/ode_steppers/ode_all_explicit_schemes_varying_mass_matrix_correctness_eigen.cc

required from ‘auto pressio::ode::create_forward_euler_stepper(Args&& ...) [with Args = {MyApp2NoMM&}]’
//tests/functional_small/ode_steppers/ode_all_explicit_schemes_varying_mass_matrix_correctness_eigen.cc:163:3: required from here
/pressio/include/pressio/./type_traits/nested_typedef_detection.hpp:59:21: error: ‘using mass_matrix_type = Eigen::MatrixXd’ is private within this context
std::enable_if_t< !std::is_void<typename T::NAME##_type>::value > \