Fix wxMSW build with wxUSE_UXTHEME==0

Make wxUxThemeIsActive() available even in this case to fix compilation
in some places and add the unavoidable preprocessor checks in other
ones.

Closes #18207.

include/wx/aui/tabart.h

@@ -318,7 +318,7 @@ protected:
         #define wxHAS_NATIVE_TABART
         #include "wx/aui/tabartgtk.h"
         #define wxAuiDefaultTabArt wxAuiGtkTabArt
-    #elif defined(__WXMSW__)
+    #elif defined(__WXMSW__) && wxUSE_UXTHEME
         #define wxHAS_NATIVE_TABART
         #include "wx/aui/tabartmsw.h"
         #define wxAuiDefaultTabArt wxAuiMSWTabArt