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.

src/msw/menuitem.cpp

@@ -165,7 +165,10 @@ class MenuDrawData
 public:
     // Wrapper around standard MARGINS structure providing some helper
     // functions and automatically initializing the margin fields to 0.
-    struct Margins : MARGINS
+    struct Margins
+#if wxUSE_UXTHEME
+        : MARGINS
+#endif // wxUSE_UXTHEME
     {
         Margins()
         {
@@ -193,6 +196,12 @@ public:
             rect.right += cyTopHeight;
             rect.bottom += cyBottomHeight;
         }
+
+#if !wxUSE_UXTHEME
+        // When MARGINS struct is not available, we need to define the fields
+        // we use ourselves.
+        int cxLeftWidth, cxRightWidth, cyTopHeight, cyBottomHeight;
+#endif // !wxUSE_UXTHEME
     };
 
     Margins ItemMargin;         // popup item margins