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.

this was done because main got rather long meanwhile.