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/msw/uxtheme.h

@@ -13,6 +13,8 @@
 
 #include "wx/defs.h"
 
+#if wxUSE_UXTHEME
+
 #include "wx/msw/private.h"     // we use GetHwndOf()
 #include <uxtheme.h>
 
@@ -253,5 +255,11 @@ private:
     wxDECLARE_NO_COPY_CLASS(wxUxThemeHandle);
 };
 
+#else // !wxUSE_UXTHEME
+
+inline bool wxUxThemeIsActive() { return false; }
+
+#endif // wxUSE_UXTHEME/!wxUSE_UXTHEME
+
 #endif // _WX_UXTHEME_H_