Add a new function to make CFEngine simple emulate "make"

During continuous deployment, it is useful to write build scripts in a self-repairing language. This makes a simple approach with class tags that can be used in any container:

classes:
  "stage1_ok" expression => makefrom("$(path)/FixIndex","$(path)/FixIndex.c");
  "stage2_ok" expression => makefrom("$(that).dvi","$(that).tex");
  "stage3_ok" expression => makefrom("$(that).html","$(that).tex");
  "stage4_ok" expression => makefrom("$(that).mobi","$(that).html");

commands:

 stage1_ok::

  "$(bin)/gcc -o FixIndex FixIndex.c" 
     contain => build_constraints,
     comment => "4. Make sure the custom patch tool is built";

etc