modules.mk:  Use depend_files function to generate depend file name
variables.mk:  Made the SCAFFOLD_INCDIRS comment more clear
variables.mk:  Append to SCAFFOLD_CXX_SUFFIXES

* This allows top level Makefiles to add more suffixes.
Changed object file naming

* Generate object file names by appending .o to the source file name.
  This allows for C++ programs that have C++ and C source files with
  the same name but different suffixes.  It also removes the need for
  language specific suffix variables.
commands.mk:  Clarify gxx comments
Add alias attribute for libraries and programs:

* The alias attribute allows one to specify a phony target that can be
  used to build a target.