Enable cpp preprocessor flag for dependencies

[zoziha]

• Use logical :: has_cpp to handle the cpp preprocessor settings for dependency occurrences, thus activating cpp flags;
• Modify subroutine set_preprocessor_flags to set_cpp_preprocessor_flags
• Add a example package: preprocess_cpp_deps;
• Fix Dependency-only cpp preprocessor settings do not work #782.

Description

All dependencies are accessed in the do loop, so check whether there are any cpp preprocessor settings in the loop.

[LKedward]

Thanks for putting together a fix so quickly @zoziha! Changes look good to me, but I'm not so familiar with this code so I will defer to @arteevraina for a full review

[arteevraina]

This looks good to me and works fine as well. 🚀
Thanks @zoziha

[zoziha]

Thank you @LKedward @arteevraina for your patient review :)

[zoziha]

With 2 approvals, I'll merge this PR: It fixes a bug of preprocess.cpp, it's good to be part of fpm-0.7.0.