[macOS] Linking fails when cmake -DBUILD_SHARED_LIBS=ON is used

[mkoeppe]

[bliss-0.77] [100%] Linking CXX executable bliss
[bliss-0.77] Undefined symbols for architecture x86_64:
[bliss-0.77]   "bliss::AbstractGraph::canonical_form(bliss::Stats&, std::__1::function<void (unsigned int, unsigned int const*)> const&, std::__1::function<bool ()> const&)", referenced from:
[bliss-0.77]       _main in bliss.cc.o
[bliss-0.77]   "bliss::AbstractGraph::set_verbose_file(__sFILE*)", referenced from:
[bliss-0.77]       _main in bliss.cc.o
[bliss-0.77]   "bliss::AbstractGraph::set_verbose_level(unsigned int)", referenced from:
[bliss-0.77]       _main in bliss.cc.o
[bliss-0.77]   "bliss::AbstractGraph::find_automorphisms(bliss::Stats&, std::__1::function<void (unsigned int, unsigned int const*)> const&, std::__1::function<bool ()> const&)", referenced from:
[bliss-0.77]       _main in bliss.cc.o
[bliss-0.77]   "bliss::print_permutation(__sFILE*, unsigned int, unsigned int const*, unsigned int)", referenced from:
[bliss-0.77]       _main in bliss.cc.o
[bliss-0.77]       std::__1::__function::__func<main::$_0, std::__1::allocator<main::$_0>, void (unsigned int, unsigned int const*)>::operator()(unsigned int&&, unsigned int const*&&) in bliss.cc.o
[bliss-0.77]   "bliss::Graph::read_dimacs(__sFILE*, __sFILE*)", referenced from:
[bliss-0.77]       _main in bliss.cc.o
[bliss-0.77]   "bliss::Digraph::read_dimacs(__sFILE*, __sFILE*)", referenced from:
[bliss-0.77]       _main in bliss.cc.o
[bliss-0.77] ld: symbol(s) not found for architecture x86_64
[bliss-0.77] clang: error: linker command failed with exit code 1 (use -v to see invocation)