CMake/fl_add_library.cmake: set target_compile_features(.. cxx_std_NN)
which is inherited by consumers of the library.
This prevents CMake user projects from failing if they would
otherwise compile with an older C++ standard.
CMake/fl_debug_var.cmake: add INTERFACE_COMPILE_FEATURES to fl_debug_target()
CMake/fl_summary.cmake: update summary format for longer variable names
CMakeLists.txt:
- check CMAKE_CXX_STANDARD given by the user (commandline)
and set C++ standard related CMake variables in the CMake cache.
The latter is experimental: setting in the cache may be reverted.
- add C++ standard related info to configure summary.
- simplify and unify build option output
- alignment can be changed in only 2 macros in CMake/fl_summary.cmake
- output system library location of image libraries if applicable
