Make build-webkit support targeting the shipping OS even if building from the beta OS OpenSource developers who have installed a beta of the non-released OS X cannot build Webkit because the binary WebKitSystemInterface is not yet available for the OS. E.g., those running the El Capitan betas right now cannot build, because the El Capitan WKSI binary is not yet available. WKSI is a pain, and maybe some day we can mostly move away from it. But for now, it should be possible to update build-webkit to target the shipping OS even when building on the pre-release OS. E.g., those running the El Capitan (10.11) beta should be able to build WebKit for Yosemite (10.10) and link to the Yosemite WKSI binary. This is something that is probably possible today with the Makefiles by messing with TARGET_MAC_OS_X_VERSION_MAJOR, but making it native to build-webkit would be a huge help. It may or may not be possible to update the Xcode project files to make this a simple option.
The nightly build process includes building for an older OS version from a current system. The build script for the nightlies should reveal which settings need to be overridden for this to work in general. Further tweaks may then be needed to accommodate prerelease OS versions.