[meson] export and use meson options as C/C++ defines

All options in meson_options.txt are exported in emper-config.h as uppercase defines and prefixed with EMPER_. Every source file using one of the EMPER_* defines now includes "emper-config.h".

emper/strategies/laws/LawsDispatcher.cpp

 #include "LawsDispatcher.hpp"
 
+#include "emper-config.h"
 #include "Runtime.hpp"
 #include "LawsStrategy.hpp"