cmLocalGenerator: Clarify GetIncludeFlags signature

Make the `config` argument non-optional so all callers must be explicit.
Convert the path style argument to an enumeration to make its role clear
at call sites.

The path style argument is implemented by `ConvertToIncludeReference`,
which was introduced with the Ninja generator by commit 5b114c9b
(Introduce a cmLocalGenerator::ConvertToIncludeReference function,
2011-09-07, v2.8.7~187^2~4).  Its only purpose is to allow the Ninja
generator to use relative paths in `-I` flags.  Add a comment explaining
this role.