optional dependencies

[Alexandre Duret-Lutz]

Hi people,

Say I am able to construct a file F.OUT, from a file 
F.IN and another file F.OPT that is optional.  That is, 
I can always build F.OUT from F.IN but the presence of 
F.OPT can specify additional constraints on the built 
and I don't want that make try to create OPT if it is 
missing.

I can write such a rule as:

F.OUT : F.IN $(wildcard F.OPT)
        construct $@

A bit tricky, but it does works.  Unfortunatly, I can't 
specify pattern rules in the same way:

%.OUT : %.IN $(wildcard %.OPT)   # won't work
        construct $@

%.OUT : %.IN $(wildcard $*.OPT)  # won't work either
        construct $@

I have to be a bit more tricky:

ifdef OPT_FILE
%.OUT : %.IN $(filter-out DUMMY, $(OPT_FILE))
        construct $@
else
FORCE :

%.OUT : FORCE
        make $@ OPT_FILE="$(wildcard $@.OPT) DUMMY"
endif

Yet, this requires an additional runs of make for each .OUT file 
that should be built, even if the child make have nothing to
do because the target is up to date.

It would be nice if there could be a way to get make 
support such optional dependencies (or does it already ?
maybe I missed something).  However I have no idea what 
syntax could be dedicated to theses.

Regards,
-- 
Alexandre Duret-Lutz