cmake/FindGSS: fix processing C header path options

cmake/FindGSS: fix processing C header path options

When processing `--cflags` received from `krb5-config` for `gssapi`:

- fix to not break on multiple `-I` options. Before this patch only
  the first `-I` option was processed as a header directory, subsequent
  ones ended up in C flags as a raw directory, without the `-I` arg.
  Follow-up to 558814e16d84aa202c5ccc0c8108a9d728e77a58

- fix to not duplicate C flags.
  Regression from 146759716cbacfd453b9fb13d1096f0595424a6c #14430

- drop local variable `_val` by re-using `_flag`.

- tidy up comments.

Ref: https://github.com/curl/curl/issues/17802#issuecomment-3029455984

Closes #17805