To support building on Mojave, there is a special makefile ( mk.x86-darwin18 ) for the runtime system and the config/install.sh uses this makefile when necessary. This makefile expects that the MacOSX10.13.sdk directory from Xcode 9 has been copied into the Xcode 10 SDKs directory. Note that updating Xcode from the AppStore will remove the 10.13 SDK, so you should keep a copy in a safe place.