Makefile: make the "sparse" target non-.PHONY

Change the "sparse" target and its *.sp dependencies to be non-.PHONY. Before this change "make sparse" would take ~5s to re-run all the *.c files through "cgcc", after it it'll create an empty *.sp file sitting alongside the *.c file, only if the *.c file or its dependencies are newer than the *.sp is the *.sp re-made. We ensure that the recursive dependencies are correct by depending on the *.o file, which in turn will have correct dependencies by either depending on all header files, or under "COMPUTE_HEADER_DEPENDENCIES=yes" the headers it needs. This means that a plain "make sparse" is much slower, as we'll now need to make the *.o files just to create the *.sp files, but incrementally creating the *.sp files is *much* faster and less verbose, it thus becomes viable to run "sparse" along with "all" as e.g. "git rebase --exec 'make all sparse'". On my box with -j8 "make sparse" was fast before, or around 5 seconds, now it only takes that long the first time, and the common case is <100ms, or however long it takes GNU make to stat the *.sp file and see that all the corresponding *.c file and its dependencies are older. See 0bcd9ae85d7 (sparse: Fix errors due to missing target-specific variables, 2011-04-21) for the modern implementation of the sparse target being changed here. It is critical that we use -Wsparse-error here, otherwise the error would only show up once, but we'd successfully create the empty *.sp file, and running a second time wouldn't show the error. I'm therefore not putting it into SPARSE_FLAGS or SP_EXTRA_FLAGS, it's not optional, the Makefile logic won't behave properly without it. Appending to $@ without a move is OK here because we're using the .DELETE_ON_ERROR Makefile feature. See 7b76d6bf221 (Makefile: add and use the ".DELETE_ON_ERROR" flag, 2021-06-29). GNU make ensures that on error this file will be removed. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>