add CI testing of FreeBSD 13 using shared libraries

In the latest release, this caused link problems:

  ld: error: undefined hidden symbol: rumur::parse(std::__1::basic_istream<char,
    std::__1::char_traits<char> >&)
  >>> referenced by main.cc
  >>>               murphi2c/CMakeFiles/murphi2c.dir/src/main.cc.o:(main)

  ld: error: undefined hidden symbol: rumur::resolve_symbols(rumur::Model&)
  >>> referenced by main.cc
  >>>               murphi2c/CMakeFiles/murphi2c.dir/src/main.cc.o:(main)

  ld: error: undefined hidden symbol: rumur::validate(rumur::Node const&)
  >>> referenced by main.cc
  >>>               murphi2c/CMakeFiles/murphi2c.dir/src/main.cc.o:(main)

  ld: error: undefined hidden symbol: rumur::sanitise_rule_names(rumur::Node&)
  >>> referenced by main.cc
  >>>               murphi2c/CMakeFiles/murphi2c.dir/src/main.cc.o:(main)

Github: related to #220 “2021.08.28: Fails to link: error: undefined hidden…”