Merge branch 'sn/osxkeychain-rust-universal'

The build system has been updated to support building universal macOS binaries when 'Rust' is enabled, by compiling separate static archives for each target triple listed in 'RUST_TARGETS' and combining them using the macOS 'lipo' tool. The 'git-credential-osxkeychain' helper has been updated to link against '$(RUST_LIB)' when 'Rust' is enabled. * sn/osxkeychain-rust-universal: contrib: wire up osxkeychain in contrib/Makefile on macOS Makefile: support universal macOS builds via RUST_TARGETS Makefile: add $(RUST_LIB) prerequisite to osxkeychain