Makefile: make DC_SHA1 the default

Makefile: make DC_SHA1 the default

We used to use the SHA1 implementation from the OpenSSL library by
default.  As we are trying to be careful against collision attacks
after the recent "shattered" announcement, switch the default to
encourage people to use DC_SHA1 implementation instead.  Those who
want to use the implementation from OpenSSL can explicitly ask for
it by OPENSSL_SHA1=YesPlease when running "make".

Signed-off-by: Junio C Hamano <gitster@pobox.com>