Merge branch 'sn/osxkeychain-rust-universal'

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