[ale] Using dependency files with GCC

[John Mills]

ALErs -

I'm bringing a legacy 'Makefile' up to date, in particular replacing
'makedepend' with 'gcc -MM' to create a dependency file: 'depend.mak'.

My rule produces a healthy looking dependency file, but I don't see how to
instruct 'make' to use the dependency rules in the generated file.

How can I pull in the dependency file so the rules are applied in a build? 

TIA.

 -- John Mills