* Work around ARM ld bug by linking separate object files.
  Arm tools don't always like ar archives, also keeps .o files for inspection.
* Add sketch dir to begining of include search path to allow per-sketch config.
  This allows customizations for libraries and the ability to tune.
  I reccommend a file names 'sketch_settings.h' for which other headers can
  obtain any customized macros and definitions.
* Bugfix: A change in the sketch Makefile won't be seen.
  Always rebuild all object files.