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a BOOTSTRAP_ELF Makefile variable that, when set (e.g. in the environment), builds a targeted-to-ELF toolchain without requiring OBJECT_FMT to be set to "ELF" (which affects how e.g. libraries are built). This basically means you don't need fake crtbegin.o and crtend.o files to be present in /usr/lib while bootstrapping the toolchain.