Fix Skew factors (#12555)

- Ensure Skew factors are calculated from test square measurements

Marlin/src/inc/Conditionals_post.h

   #define _SKEW_FACTOR(a,b,c) _SKEW_SIDE(float(a),_GET_SIDE(float(a),float(b),float(c)),float(c))
 
   #ifndef XY_SKEW_FACTOR
-    constexpr float XY_SKEW_FACTOR = (
-      #if defined(XY_DIAG_AC) && defined(XY_DIAG_BD) && defined(XY_SIDE_AD)
-        _SKEW_FACTOR(XY_DIAG_AC, XY_DIAG_BD, XY_SIDE_AD)
-      #else
-        0.0
-      #endif
-    );
+    #if defined(XY_DIAG_AC) && defined(XY_DIAG_BD) && defined(XY_SIDE_AD)
+      #define XY_SKEW_FACTOR _SKEW_FACTOR(XY_DIAG_AC, XY_DIAG_BD, XY_SIDE_AD)
+    #else
+      #define XY_SKEW_FACTOR 0.0
+    #endif
   #endif
   #ifndef XZ_SKEW_FACTOR
     #if defined(XY_SIDE_AD) && !defined(XZ_SIDE_AD)
       #define XZ_SIDE_AD XY_SIDE_AD
     #endif
-    constexpr float XZ_SKEW_FACTOR = (
-      #if defined(XZ_DIAG_AC) && defined(XZ_DIAG_BD) && defined(XZ_SIDE_AD)
-        _SKEW_FACTOR(XZ_DIAG_AC, XZ_DIAG_BD, XZ_SIDE_AD)
-      #else
-        0.0
-      #endif
-    );
+    #if defined(XZ_DIAG_AC) && defined(XZ_DIAG_BD) && defined(XZ_SIDE_AD)
+      #define XZ_SKEW_FACTOR _SKEW_FACTOR(XZ_DIAG_AC, XZ_DIAG_BD, XZ_SIDE_AD)
+    #else
+      #define XZ_SKEW_FACTOR 0.0
+    #endif
   #endif
   #ifndef YZ_SKEW_FACTOR
-    constexpr float YZ_SKEW_FACTOR = (
-      #if defined(YZ_DIAG_AC) && defined(YZ_DIAG_BD) && defined(YZ_SIDE_AD)
-        _SKEW_FACTOR(YZ_DIAG_AC, YZ_DIAG_BD, YZ_SIDE_AD)
-      #else
-        0.0
-      #endif
-    );
+    #if defined(YZ_DIAG_AC) && defined(YZ_DIAG_BD) && defined(YZ_SIDE_AD)
+      #define YZ_SKEW_FACTOR _SKEW_FACTOR(YZ_DIAG_AC, YZ_DIAG_BD, YZ_SIDE_AD)
+    #else
+      #define YZ_SKEW_FACTOR 0.0
+    #endif
   #endif
 #endif // SKEW_CORRECTION
 

Marlin/src/module/planner.h

         min_travel_feedrate_mm_s;               // (mm/s) M205 T - Minimum travel feedrate
 } planner_settings_t;
 
-#ifndef XY_SKEW_FACTOR
+#if DISABLED(SKEW_CORRECTION)
   #define XY_SKEW_FACTOR 0
-#endif
-#ifndef XZ_SKEW_FACTOR
   #define XZ_SKEW_FACTOR 0
-#endif
-#ifndef YZ_SKEW_FACTOR
   #define YZ_SKEW_FACTOR 0
 #endif