Merge branch 'bc/sha1-header-selection-with-cpp-macros'

Our source code has used the SHA1_HEADER cpp macro after "#include"
in the C code to switch among the SHA-1 implementations. Instead,
list the exact header file names and switch among implementations
using "#ifdef BLK_SHA1/#include "block-sha1/sha1.h"/.../#endif";
this helps some IDE tools.

* bc/sha1-header-selection-with-cpp-macros:
  hash.h: move SHA-1 implementation selection into a header file

diff --cc Makefile
Simple merge

diff --cc cache.h
Simple merge