- assume C++ standard-conforming environment
- add a user-option for gnuplot-support
- separate gnuplot-code into declaration and implementation,
so we can define at EO-build-time whether to use it or not.
Adopt code and Makefiles to above changes.
Some minor fixes.
Moreover, I got fed up with error due to const/non const, so I modified
PipeComSend to take a const argument. THe consequence is that it will not run
under plain C any more.