Warning about non-virtual destructor

[stertingen]

Building the code with -Wnon-virtual-dtor emits warnings in lyra/detail/deprecated_parser_customization.hpp.

I propose at least one of two things to fix this:

1. Add a virtual default destructor to 'lyra::parser_customization', fixing the actual issue
2. Change CMakeLists.txt to mark include directories as SYSTEM includes, avoiding warnings when using via add_subdirectory()

I can provide a PR if desired.

Edit: This would be fixed by merging https://github.com/bfgroup/Lyra/pull/80