Define __VISUALC__ for ICC under Windows again.

Define __VISUALC__ for ICC under Windows again.

During the refactoring of r74496, the logic of the check for Intel compiler
was slightly altered resulting in not defining __VISUALC__ for it any longer
which broke compilation with it.

Restore this definition now to fix it, even though it could admittedly be
better to explicitly check for __INTELC__ in the places where we currently
only check for __VISUALC__ and reserve the latter only for the case when we
are really using MSVC.

Closes #15359.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74640 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775