OBJDIR=, SRCDIR=, LIB= and INCLUDE= directives are checked for
      validity.

      In the case of the OBJDIR= and SRCDIR= directives, if an
      invalid path is used, the make script will halt with an
      appropriate error message.

      In the case of the INCLUDE= and LIB= directives, the existance of
      each file is validated. The make script will halt with an
      appropriate error message when it comes to an invalid one.