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first target built, GIT-CFLAGS will appear to be out of date,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>

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