Fix conanfile.py for new Conan versions
Fix conanfile.py for new Conan versions

One of the latest Conan versions has changed the way CMake generators
work, and header-only libraries now need to have a proper settings field
in order to work properly.

[ci skip]

Morwenn/cpp-sort,Morwenn/cpp-sort,Morwenn/cpp-sort,Morwenn/cpp-sort