error on non-existing targets in dependency list
error on non-existing targets in dependency list

Added detection and error when TARGET_DEPENDENDS in the rules.mk of any target refers to a non-existing target.
Sample error message:
  cxxblocks-master/build/example/makefile:121: *** unknown targets [boost] in dependency list of the target [generator].  Stop.
mrbald/cxxblocks,mrbald/cxxblocks