Makefile: use target-specific variable to pass flags to cc

Makefile: use target-specific variable to pass flags to cc

This allows reusing the standard %.o: %.c pattern rule even for
targets that require special flags to be set.  Thus after this
change, any changes in the command for compilation only have to
be performed in one place.

Target-specific variables have been supported in GNU make since
version 3.77, which has been available since 1998.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>