cpp seems not to include # line "file" output for empty files

[E. Jay Berkenbilt]

   > It appears that cpp is deciding that it doesn't have to generate the #
   > line output if there is no output from the file.  This makes sense for
   > the intend use of these directives for telling the compiler what
   > filename/line number a line of cpp output came from, but it badly
   > hoses dependency generators and other programs that rely on this
   > output for getting a list of files that cpp read.  Such functionality
   > is essential for writing portable dependency generators.  I feel like
   > I've read this in the ISO standard but I can't seem to find any
   > documentation that firmly states that ISO requires this behavior.

>   If you want dependency info from cpp, use the switches to tell it to
>   give you dependency info.

Not portable.

>   Also, if the file generates no output, then a.c really wasn't dependent
>   on it.

Wrong -- see my example.  a.c is most definitely dependent upon a.h.