configure: Separate commonalities in msvc and icl flags

Allows for easier handling of flags that may be specific to icl or msvc.
Furthermore, simplify the handling of warnings and remarks thanks to
icl's support of -Wall on Windows.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
configure: icl: Merge -Qdiag-error parameters

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>