Makefile: add option to disable automatic dependency generation

Now that the COMPUTE_HEADER_DEPENDENCIES feature is turned on
automatically for compilers that support it (see v1.7.8-rc0~142^2~1,
2011-08-18), there is no easy way to force it off.  For example,
setting COMPUTE_HEADER_DEPENDENCIES to the empty string in config.mak
just tells the makefile to treat it as undefined and run a test
command to see if the -MMD option is supported.

So allow setting COMPUTE_HEADER_DEPENDENCIES=no to explicitly force
the feature off.  The new semantics:

 - "yes" means to explicitly enable the feature
 - "no" means to disable it
 - "auto" means to autodetect

The default is still "auto".  Any value other than these three will
cause the build to error out with a descriptive message so typos and
stale settings in config.mak don't result in mysterious behavior.

	Makefile:1278: *** please set COMPUTE_HEADER_DEPENDENCIES to
	yes, no, or auto (not "1").  Stop.

So now when someone using a compiler without -MMD support reports
trouble building git, you can reproduce it by running "make
COMPUTE_HEADER_DEPENDENCIES=no".

Suggested-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Tested-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -250,6 +250,12 @@ all::
 #   DEFAULT_EDITOR='$GIT_FALLBACK_EDITOR',
 #   DEFAULT_EDITOR='"C:\Program Files\Vim\gvim.exe" --nofork'
 #
+# Define COMPUTE_HEADER_DEPENDENCIES to "yes" if you want dependencies on
+# header files to be automatically computed, to avoid rebuilding objects when
+# an unrelated header file changes.  Define it to "no" to use the hard-coded
+# dependency rules.  The default is "auto", which means to use computed header
+# dependencies if your compiler is detected to support it.
+#
 # Define CHECK_HEADER_DEPENDENCIES to check for problems in the hard-coded
 # dependency rules.
 #
@@ -1246,21 +1252,32 @@ endif
 endif
 
 ifdef CHECK_HEADER_DEPENDENCIES
-COMPUTE_HEADER_DEPENDENCIES =
+COMPUTE_HEADER_DEPENDENCIES = no
 USE_COMPUTED_HEADER_DEPENDENCIES =
-else
+endif
+
 ifndef COMPUTE_HEADER_DEPENDENCIES
+COMPUTE_HEADER_DEPENDENCIES = auto
+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; \
 	echo $$?)
 ifeq ($(dep_check),0)
-COMPUTE_HEADER_DEPENDENCIES=YesPlease
-endif
+override COMPUTE_HEADER_DEPENDENCIES = yes
+else
+override COMPUTE_HEADER_DEPENDENCIES = no
 endif
 endif
 
-ifdef COMPUTE_HEADER_DEPENDENCIES
+ifeq ($(COMPUTE_HEADER_DEPENDENCIES),yes)
 USE_COMPUTED_HEADER_DEPENDENCIES = YesPlease
+else
+ifneq ($(COMPUTE_HEADER_DEPENDENCIES),no)
+$(error please set COMPUTE_HEADER_DEPENDENCIES to yes, no, or auto \
+(not "$(COMPUTE_HEADER_DEPENDENCIES)"))
+endif
 endif
 
 ifdef SANE_TOOL_PATH
@@ -1907,7 +1924,7 @@ OBJECTS := $(GIT_OBJS) $(XDIFF_OBJS) $(VCSSVN_OBJS)
 dep_files := $(foreach f,$(OBJECTS),$(dir $f).depend/$(notdir $f).d)
 dep_dirs := $(addsuffix .depend,$(sort $(dir $(OBJECTS))))
 
-ifdef COMPUTE_HEADER_DEPENDENCIES
+ifeq ($(COMPUTE_HEADER_DEPENDENCIES),yes)
 $(dep_dirs):
 	@mkdir -p $@
 
@@ -1920,7 +1937,7 @@ Please unset CHECK_HEADER_DEPENDENCIES and try again)
 endif
 endif
 
-ifndef COMPUTE_HEADER_DEPENDENCIES
+ifneq ($(COMPUTE_HEADER_DEPENDENCIES),yes)
 ifndef CHECK_HEADER_DEPENDENCIES
 dep_dirs =
 missing_dep_dirs =