title: Out of tree builds with makefiles and static pattern rules
body: <p>I'm working on some bare-metal embedded code that runs on ARM, and thus has to deal with the whole ARM vs. THUMB mode distinction. The current build system uses static pattern rules to determine whether to compile files in ARM or THUMB mode. </p>
  
  <pre><code>$(ACOBJS) : %.o : %.c
      @echo
      $(CC) -c $(CFLAGS) $(AOPT) -I . $(IINCDIR) $&lt; -o $@
  $(TCOBJS) : %.o : %.c
      @echo
      $(CC) -c $(CFLAGS) $(TOPT) -I . $(IINCDIR) $&lt; -o $@
  </code></pre>
  
  <p>Where ACOBJS is a list of output objects that should be in ARM mode and the same for TCOBJS and Thumb mode. These lists are created from the list of sources in the usual manner of</p>
  
  <pre><code>ACOBJS   = $(ACSRC:.c=.o)
  TCOBJS   = $(TCSRC:.c=.o)
  </code></pre>
  
  <p>Currently this results in the object files from the build being strewn about the source tree, which I don't particularly desire. I've been trying to set this up for out of tree builds but haven't been able to get this to work. I don't necessarily need to get full out of tree builds working, but I would like to at least be able to use an output directory under which all the intermediate files end up going. What is the best strategy to achieve this under these constraints?</p>
  
  <p>One option I'm considering is using either automake or the whole autotools toolchain to build a makefile. This would seem to support creating the type of makefile I want, but seems like overkill. It also seems like there would be an inherent impedance mismatch between autotools, which is designed for portable builds, and bare-metal embedded systems, where things like host tuple are dictated by the target micro. </p>