title: reevaluate makefile variables
body: <p>Is there a way to reevaluate a variable's definition upon each use? For example:</p>
  
  <pre><code>MAP_FILES = $(shell find $(TMP) -name "*.map")
  
  all: generate_map_files work_with_map_files
  
  generate_map_files:
     ./map-builder
  
  work\_with\_map_files: $(MAP_FILES)
     ./map-user
  
  %.map:
     ./map-edit $@
  </code></pre>
  
  <p>So, MAP_FILES will be evaluated when the makefile is read, and if there are no .map files in the directory $TMP the variable will be empty.  However after the generate_map_files rule is completed there will be .map files in the directory and I would like the list of those .map files to be prerequisites to the work_with_map_files rule. </p>
  
  <p>I don't know the filenames of the .map files before they are generated so I can not declare a variable with filenames explicitly.  I need the variable to be set with the list of map files once they have been generated. Any suggestions would be very helpful. Thanks.</p>