Makefile: Fix occasional GIT-CFLAGS breakage

GNU make’s target-specific variables facility has one weird facet: any variables set for a given target apply to all of its dependencies, too. For example, when running “make exec_cmd.o”, since exec_cmd.o depends on GIT-CFLAGS, the variable assignment in exec_cmd.s exec_cmd.o: ALL_CFLAGS += \ '-DGIT_EXEC_PATH="$(gitexecdir_SQ)"' \ '-DBINDIR="$(bindir_relative_SQ)"' \ '-DPREFIX="$(prefix_SQ)"' applies when refreshing GIT-CFLAGS, and the extra options get included in the tracked compiler flags. If an object file like this is the first target built, GIT-CFLAGS will appear to be out of date, resulting in useless rebuilds and the dreaded “new build flags or prefix” message. This does not happen with every build because GIT-CFLAGS is only refreshed once in a given “make” run, and usually the first target does not set any variables. When this problem does rear its head, it is very annoying. So put target-specific flags in a separate EXTRA_CPPFLAGS variable that is not included in $(TRACK_CFLAGS). Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>