The GNU Make "right" way
In GNU Make syntax there is really only one correct way to get a single rule with multiple outputs, and that is to use a pattern rule:
all: parser.h parser.c
%.h %.c: %.i
@echo Generating $*.h and $*.c from $*.i
@touch $*.c $*.h
In direct contrast to the first example, this actually will create a single rule that creates two outputs. If you run with gmake -j 2, you'll see the files are updated only once:
Generating parser.h and parser.c from parser.i
This is the only construct that produces the correct dependency graph and behavior. Unfortunately, it has one significant shortcoming: It requires that the input and all the outputs share a common stem, such as parser in our simple examples, so it's not as flexible as we'd like it to be. Still, if your files do fit this restriction, then this is the best solution for you.
Now you know some of the ways to create a makefile that generates multiple outputs from a single command. If possible, you should use a pattern rule with multiple outputs. If that doesn't work for you, hopefully one of the alternatives I've shown you will.
About the Author
Eric Melski was part of the team that founded Electric Cloud and is now Architect for ElectricAccelerator. Before Electric Cloud, he was a Software Engineer at Scriptics, Inc. and Interwoven. He holds a BS in Computer Science from the University of Wisconsin.