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>