Merge branch 'ab/sha-makefile-doc'

Makefile comments updates and reordering to clarify knobs used to choose SHA implementations. * ab/sha-makefile-doc: Makefile: discuss SHAttered in *_SHA{1,256} discussion Makefile: document default SHA-1 backend on OSX Makefile & test-tool: replace "DC_SHA1" variable with a "define" Makefile: document SHA-1 and SHA-256 default and selection order Makefile: document default SHA-256 backend Makefile: rephrase the discussion of *_SHA1 knobs Makefile: create and use sections for "define" flag listing Makefile: correct DC_SHA1 documentation INSTALL: remove discussion of SHA-1 backends Makefile: always (re)set DC_SHA1 on fallback