R – GNU make with many target directories


I have to integrate the generation of many HTML files in an existing Makefile.
The problem is that the HTML files need to reside in many different directories.
My idea is to write an implicit rule that converts the source file (*.st) to the corresponding html file

%.html: %.st
    $(HPC) -o $@ $<

and a rule that depends on all html files

all: $(html)

If the HTML file is not in the builddir, make doesn't find the implicit rule: *** No rule to make target.
If I change the implicit rule like so

$(rootdir)/build/doc/2009/06/01/%.html: %.st  
    $(HPC) -o $@ $<

it's found, but then I have to have an implicit rule for nearly every file in the project.
According to Implicit Rule Search Algorithm in the GNU make manual, rule search works like this:

  1. Split the entire target name t into a directory part, called d, and the rest, called n. For
    example, if t is src/foo.o,
    then d is src/,
    and n is foo.o.
  2. Make a list of all the pattern rules one of whose targets matches t or n.
    If the target pattern contains a slash,
    it is matched against t;
    otherwise, against n.

Why is the implicit rule not found, and what would be the most elegant solution, assuming GNU make is used?

Here is a stripped down version of my Makefile:

rootdir  = /home/user/project/doc
HPC      = /usr/local/bin/hpc

html = $(rootdir)/build/doc/2009/06/01/some.html

%.html: %.st
    $(HPC) -o $@ $<

#This works, but requires a rule for every output dir
#$(rootdir)/build/doc/2009/06/01/%.html: %.st  
#   $(HPC) -o $@ $<

.PHONY: all
all: $(html)

Best Solution

Like Maria Shalnova I like recursive make (though I disagree with "Recursive Make Considered Harmful"), and in general it's better to make something HERE from a source THERE, not the reverse. But if you must, I suggest a slight improvement: have generateHtml generate only the RULE, not the COMMANDS.

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