Unable to build on FreeBSDv11.2 Release

After following the instructions for building on FreeBSD in `doc/build-unix.md`, gmake fails with the following error:
```
/usr/local/bin/ld: warning: libcrypto.so.8, needed by /usr/local/lib/libevent_pthreads.so, may conflict with libcrypto.so.9
libbitcoin_util.a(libbitcoin_util_a-util.o): In function `SetupEnvironment()':
/home/user/Code/bitcoin-abc/build-aux/src/.././../src/util.cpp:617: undefined reference to `boost::filesystem::path::imbue(std::locale const&)'
/home/user/Code/bitcoin-abc/build-aux/src/.././../src/util.cpp:618: undefined reference to `boost::filesystem::path::imbue(std::locale const&)'
libbitcoin_util.a(libbitcoin_util_a-util.o): In function `boost::program_options::detail::basic_config_file_iterator::getline(std::__cxx11::basic_string, std::allocator >&)':
/usr/local/include/boost/program_options/detail/config_file.hpp:171: undefined reference to `boost::program_options::to_internal(std::__cxx11::basic_string, std::allocator > const&)'
libbitcoin_util.a(libbitcoin_util_a-util.o): In function `boost::program_options::detail::basic_config_file_iterator::basic_config_file_iterator(std::istream&, std::set, std::allocator >, std::less, std::allocator > >, std::allocator, std::allocator > > > const&, bool)':
/usr/local/include/boost/program_options/detail/config_file.hpp:155: undefined reference to `boost::program_options::detail::common_config_file_iterator::common_config_file_iterator(std::set, std::allocator >, std::less, std::allocator > >, std::allocator, std::allocator > > > const&, bool)'
collect2: error: ld returned 1 exit status
gmake[2]: *** [Makefile:4198: bitcoind] Error 1
gmake[2]: Leaving directory '/home/user/Code/bitcoin-abc/build-aux/src'
gmake[1]: *** [Makefile:10520: all-recursive] Error 1
gmake[1]: Leaving directory '/home/user/Code/bitcoin-abc/build-aux/src'
gmake: *** [Makefile:720: all-recursive] Error 1
```
I have ensured that `boost` is installed and working.
cwd: `bitcoin-abc/build-aux` (used this as current working directory because configure was failing)
The commands I ran to produce this error were
`$ ./../autogen.sh`
`$ ./../configure --without-gui --without-miniupnpc --disable-wallet`
Both of these commands succeed. I follow them with `$ gmake` and each time the build error occurs.

### Machine specs:
- OS: Freebsd 11.2
- CPU: Intel Xeon E3-1245
- RAM: 16GB