CMake changes to avoid name conflicts

[chillenb]

This pull request does two things:

1. Rename config.h to cint_config.h in order to avoid name conflicts if another CMake project imports libcint as a module, but also has a file named config.h
2. Replace include_directories with target_include_directories, to avoid accidentally including libcint headers in a higher level build

I have not found this to break anything in PySCF, but if you spot a problem I will do my best to fix it.