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