- type of basic_option_t::_value changed from std::optional<T> to T.
- `operator bool()` now returns `_value != default_value` instead of `has_value()`.

Added new architectures and OSs:
- includes amd64 (debian 10, ubuntu 20.04, ubuntu 21.04)
- includes arm64 (debian 10, ubuntu 20.04, ubuntu 21.04)
- includes armhf (debian 10)

furhtermore, several other improvements were done:
- added new compiler warnings for gcc-10 and gcc-11 for pedantic builds
- created new build-targets (i.e. libClaPP_CXX_COMPILE_OPTIONS) which contain
  all warnings for the current compiler.

This was required, as clang-7 formatted code leads to
`compound-token-split-by-space` warning in clang-12.

src/include/clapp/parser.h: removed unused members mandatory_arguments and optional_argument from basic_parser_t

this was done because main got rather long meanwhile.