[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".
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".