Backport the following changes from HEAD:

1.270:
As an effect of revisions 1.261, BUILD_CMD was changed so $(DIRS)
wasn't respected when using it to build different parts of OpenSSL.
1.269 was an attempt to correct that, but unfortunately meant that we
built every part that was given i $(DIRS) 7 times.  This change puts
back the original intent with BUILD_CMD via the new macro
BUILD_ONE_CMD while keeping the intent with RECURSIVE_BUILD_CMD.

1.271:
Document the building macros.