First drop of package_definition, obtained from the system model used in the platform build, along with a generic SystemBuild fragment (from TB91sf)
#include "Base.xcconfig"
MACOSX_DEPLOYMENT_TARGET = $(MACOSX_DEPLOYMENT_TARGET_$(MAC_OS_X_VERSION_MAJOR));
MACOSX_DEPLOYMENT_TARGET_ = 10.4;
MACOSX_DEPLOYMENT_TARGET_1040 = 10.4;
MACOSX_DEPLOYMENT_TARGET_1050 = 10.5;