Linking errors with boost on macOS

I recently revived an old project and tried to get it to build on my Mac. I was immediately met with this error:

Undefined symbols for architecture x86_64:
  "boost::system::detail::system_category_instance", referenced from:
      boost::system::system_category() in ConsoleApp.cpp.o
      boost::system::system_category() in OAuth2Login.cpp.o
      boost::system::system_category() in RedditSession.cpp.o
      boost::system::system_category() in utils.cpp.o
  "boost::system::detail::generic_category_instance", referenced from:
      boost::system::generic_category() in arcc.cpp.o
      boost::system::generic_category() in CommandHistory.cpp.o
      boost::system::generic_category() in ConsoleApp.cpp.o
      boost::system::generic_category() in OAuth2Login.cpp.o
      boost::system::generic_category() in RedditSession.cpp.o
      boost::system::generic_category() in Settings.cpp.o
      boost::system::generic_category() in utils.cpp.o
      ...
ld: symbol(s) not found for architecture x86_64

This was perplexing because everything looked fine in my CMake files and I wasn’t getting this error on Windows or Ubuntu. The first thing I did was print verbose output to make sure I was in fact linking what I thought I was linking:

cd /Users/zethon/src/arcc/build/arcc && /usr/local/Cellar/cmake/3.18.3/bin/cmake -E cmake_link_script CMakeFiles/arcc.dir/link.txt --verbose=1
... (ommitted) ...
/lib -lboost_program_options -lboost_timer -lboost_thread -lboost_chrono -lboost_filesystem -lboost_system -lboost_stacktrace_addr2line -lboost_stacktrace_basic -lboost_stacktrace_noop -lboost_unit_test_framework -lcurl -lfmtd -lform_g -lmenu_g -lncurses++_g -lncurses_g -lpanel_g -lz /usr/local/lib/libboost_system.a /usr/local/lib/libboost_thread-mt.a /usr/local/opt/openssl/lib/libssl.dylib /usr/local/opt/openssl/lib/libcrypto.dylib

In case it’s not obvious, the functions in my error are part of the Boost System library (which I found out through a Google search). The output above proved that I definitely am linking to boost_system. So.. back to Google!

Then I found this link on StackOverflow, which had the answer:

Adding add_definitions(-DBOOST_ERROR_CODE_HEADER_ONLY) for cmake file.

I added this to my CMake and it worked!

Full admission, this is one of those answers that work, and I don’t understand why or what’s going on. All that matters is that my build is working, so I’m ready to move on.