1 # Add a user to a group from the command line
   2 sudo dseditgroup -o edit -a $username_to_add -t user $group_to_be_added_to

Cross compiling

tpoechtrager/osxcross: OS X cross toolchain for Linux, *BSD and Windows (Cygwin)

c++ - Cross-compiling from OS X to Windows using clang and the Visual Studio 2013 runtime - Stack Overflow. Mentions "-fmsc-version" flag.

Xcode/macOS platform SDK

https://github.com/phracker/MacOSX-SDKs: Archived versions of macOS SDKs.

Downloads for Apple developers: archives of Xcode, etc going back.

Code signing

How to sign your Mac OS X App for Gatekeeper: Easy-to-read high level document

http://stackoverflow.com/a/41058295/14878 — signing a .dylib shared library