Process files with m4 in makefile - makefile

Question

as a part of build process I need to process some files with m4. These files are given an '.in' extension and are located not only in the top-level dir, but also in subdirs.
I locate them using find and process them in a shell loop.
Is there some makefile syntax magic to write simpler rule to process them save the output into a file (the '.in' extension stripped) in the same directory as the input file?
This is what I have in my makefile now:
PROCESS_FILES=$(shell find . -name \*.in)
WORK_FILES=$(subst .in,,$(PROCESS_FILES))
$(WORK_FILES): $(PROCESS_FILES)
for file in $(PROCESS_FILES); \
do \
m4 $$file > $${file%.*};\
done

Answer 1

You could write a set of rules to convert files with the suffix .in to files without that suffix.
In the classic notation, that would be something like:
# Not functional — see discussion
.SUFFIXES: .in # Add .in as a suffix
M4 = m4
M4FLAGS =
M4SCRIPT = xyz.m4
.in:
${M4} ${M4FLAGS} ${M4SCRIPT} $< > $*
This adds .in as a recognized suffix, and says that you convert the file with a .in suffix to the file without it using the command specified by the three macros. The $< is the name of the file with the .in extension; the $* is the name of the file without the extension.
Unfortunately though, that notation only works when the files to be converted have names such as xyz.in and need to be converted to xyz. It does not work for a case where xyz.h.in needs to be converted to xyz.h.
It would be possible to specify that the suffixes are .h.in and .h, but then the rule for converting between the two starts with .h.in.h: and make gets confused.
However, GNU make has an alternative notation for defining suffix rules which can accommodate this:
.SUFFIXES: .h.in .h # Add .h.in and .h as suffixes
M4 = m4
M4FLAGS =
M4SCRIPT = xyz.m4
%.h: %.h.in
${M4} ${M4FLAGS} ${M4SCRIPT} $< > $*.h
The %.h: %.h.in line is able to deal with the suffix with two dots. The only nuisance is that for each extension such as .c.in or .y.in or .mk.in, you have to provide similar mappings.
(Tested on Mac OS X 10.7.4. If that makefile is xyz.mk, you can create an empty file xyz.h.in and then run make -n -f xyz.mk xyz.h and you'll see the build would run the command m4 xyz.m4 xyz.h.in > xyz.h.)

Answer 2

If you have a source file path/to/file.in, a rule for making % from %.in, and a dependency on the output file path/to/file, that should be all you need.
Perhaps you want to express your dependencies in terms of the output from find. In GNU Make this is easy; in other dialects, perhaps you want to generate and include something like find -name '*.in' | sed 's/\.in$//' >make.dep