Makefile: linux has /dev/tty

Therefore we can turn on our custom prompt function instead
of relying on getpass.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -837,6 +837,7 @@ ifeq ($(uname_S),Linux)
 	NO_STRLCPY = YesPlease
 	NO_MKSTEMPS = YesPlease
 	HAVE_PATHS_H = YesPlease
+	HAVE_DEV_TTY = YesPlease
 endif
 ifeq ($(uname_S),GNU/kFreeBSD)
 	NO_STRLCPY = YesPlease