Choosing a gcc compiler on a Linux machine for compilation with "make"

I encountered the following error message on a Linux machine while attempting to compile a program with the command "make":

build/chem/rdm.o: In function `RDM::get_2rdm_slow(std::vector<Det, std::allocator<Det> > const&, std::vector<double , std::allocator<double> > const&, Integrals const&)': rdm.cc:(.text+0x37d2): undefined reference to `Timer::checkpoint(std::__cxx11::basic_string<char, std::char_traits< char>, std::allocator<char> > const&)'

I think the compiler I am using, gcc/6.5.0, is the wrong version, but I am not completely sure that this is the issue. Does anyone have an idea of what could be going on? Makefile