Makefile: add option to disable automatic dependency generation

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>