Exclude wxMSW-specific code from compilation under other platforms.

Compilation fix after r75648, see #13328.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75651 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Use wxUSE_WXDIB in preprocessor tests.

NEVER_USE_DIB is meant to be used only in conjunction with
{SOMETIMES,ALWAYS}_USE_DIB constants, use wxUSE_WXDIB elsewhere.

This is more logical and also fixes inadvertent use of NEVER_USE_DIB before it
is defined added by the changes of r75648.

See #13328.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75652 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775