Makefile: explicitly set target name for autogenerated dependencies

"gcc -MF depfile -MMD -MP -c -o path/to/file.o" produces a makefile
snippet named "depfile" describing what files are needed to build the
target given by "-o".  When ccache versions before v3.0pre0~187 (Fix
handling of the -MD and -MDD options, 2009-11-01) run, they execute

	gcc -MF depfile -MMD -MP -E

instead to get the final content for hashing.  Notice that the "-c -o"
combination is replaced by "-E".  The result is a target name without
a leading path.

Thus when building git with such versions of ccache with
COMPUTE_HEADER_DEPENDENCIES enabled, the generated makefile snippets
define dependencies for the wrong target:

	$ make builtin/add.o
	GIT_VERSION = 1.7.8.rc3
	    * new build flags or prefix
	    CC builtin/add.o
	$ head -1 builtin/.depend/add.o.d
	add.o: builtin/add.c cache.h git-compat-util.h compat/bswap.h strbuf.h \

After a change in a header file, object files in a subdirectory are
not automatically rebuilt by "make":

	$ touch cache.h
	$ make builtin/add.o
	$

Luckily we can prevent trouble by explicitly supplying the name of the
target to ccache and gcc, using the -MQ option.  Do so.

Reported-and-tested-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reported-by: : 허종만 <jongman.heo@samsung.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1388,7 +1388,8 @@ endif
 
 ifeq ($(COMPUTE_HEADER_DEPENDENCIES),auto)
 dep_check = $(shell $(CC) $(ALL_CFLAGS) \
-	-c -MF /dev/null -MMD -MP -x c /dev/null -o /dev/null 2>&1; \
+	-c -MF /dev/null -MQ /dev/null -MMD -MP \
+	-x c /dev/null -o /dev/null 2>&1; \
 	echo $$?)
 ifeq ($(dep_check),0)
 override COMPUTE_HEADER_DEPENDENCIES = yes
@@ -2211,7 +2212,7 @@ $(dep_dirs):
 
 missing_dep_dirs := $(filter-out $(wildcard $(dep_dirs)),$(dep_dirs))
 dep_file = $(dir $@).depend/$(notdir $@).d
-dep_args = -MF $(dep_file) -MMD -MP
+dep_args = -MF $(dep_file) -MQ $@ -MMD -MP
 ifdef CHECK_HEADER_DEPENDENCIES
 $(error cannot compute header dependencies outside a normal build. \
 Please unset CHECK_HEADER_DEPENDENCIES and try again)