Only define wxUSE_WEBVIEW if wxWebView is really available.

Only define wxUSE_WEBVIEW if wxWebView is really available.

Don't define wxUSE_WEBVIEW in configure if WebKit is not available under Unix
(we should also check for wxUSE_WEBVIEW_IE under Windows later) and add the
check that either wxUSE_WEBVIEW_WEBKIT or wxUSE_WEBVIEW_IE is defined if
wxUSE_WEBVIEW is to wx/chkconf.h.

This makes it possible to just check for wxUSE_WEBVIEW instead of checking for
wxUSE_WEBVIEW && (wxUSE_WEBVIEW_IE || wxUSE_WEBVIEW_WEBKIT) as the code did
previously which was ugly and error-prone.

Also, define wxUSE_WEBVIEW_IE in configure under MSW. Currently this supposes
that the required IE headers/libraries are available which is probably wrong,
we should add checks for them later.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69108 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775