adding a third type of prerequisite

[Christof Warlich]

Hi Paul,

please apologize my beggar-mail (i.e. asking to add a new feature to 
make), but
thinking it over and over again during the last couple of days, I 
believe this could be
something of general interest for everyone using GNU make, so I'd like 
to kindly
ask to read on.

First, here is what I'd like to ask for: A third type of prerequisites 
besides the existing
two ("ordinary prerequisites" and "order-only prerequisites", let's call 
them
"dependency-only prerequisites") that, like "order-only prerequisites", 
are _not_
included in the automatic variable $^, but that behave like ordinary 
prerequisites
otherwise.

I have set up an example to understand why I think this would be useful: 
Consider
a tool that makes its targets from several files given on its command 
line, but that
may have additional dependencies that must not be passed to that tool. 
In such a
case neither of the automatic variables $< nor $^ match, as the first 
only expands
to the first prerequisite, while the latter expands to all prerequisites.

Here is the complete example that illustrates the idea:

$ cat sum_main.c
#include <stdio.h>
#include "sum.h"
int main(int argc, char *argv[]) {
    int sum = 0;
    while(argc--) sum += size(argv[argc]);
    printf("%d\n", sum);
    return 0;
}

$ cat sum_lib.c
#include <sys/stat.h>
int size(const char *file) {
    struct stat s;
    stat(file, &s);
    return s.st_size;
}

$ cat sum.h
int size(const char *file);

$ cat Makefile
SUM := ./sum
all: allCfileSizeSum.sum
%.sum: *.c *.h | ${SUM}
    ${SUM} $^ >$@
%: %_*.c
    gcc $^ -o $@
${SUM}: | sum.h

$ make
gcc sum_lib.c sum_main.c -o sum
./sum sum_lib.c sum_main.c sum.h >allCfileSizeSum.sum

Everything works fine when calling make for the first time, but you may 
have noticed
that I had to use "order-only prerequisites" to avoid that the sum 
executable is counted
as a source file and that sum.h appears as additional input file to gcc. 
Hence:

$ touch sum.h
$ make
make: Nothing to be done for `all'.

That's not the desired behaviour: As everything ultimately depends on sum.h,
everything should be rebuilt.

The suggested "dependency-only prerequisite" would gracefully solve the 
issue.

An alternative could be to introduce a new automatic variable that 
expands to
only those prerequisites that have been listed in the rule that has a 
recipe while
excluding any prerequisites from reciepe-less rules, but this solution 
would be
inferior as it doesn't solve the problem for the example's first rule .

What do you think: Does this make sense? By the way, the idea came up when
playing with David Boyce' "audited objects" tool, which implements the 
generic
dependency generation approach that you have described here:

http://mad-scientist.net/make/autodep.html#nonc

A "dependency-only prerequisite" would particularly well be suited within
this context.

Cheers,

Chris