Eliminate most warnings

- Fix a bug reading `code_value` for `M503 Sn`
- Hide and remove unused variables

Marlin/Marlin_main.cpp

     }
   }
 
-  float area = .0;
   if (code_seen('D')) {
     float diameter = code_value();
     // setting any extruder filament size disables volumetric on the assumption that
  * M503: print settings currently in memory
  */
 inline void gcode_M503() {
-  Config_PrintSettings(code_seen('S') && code_value == 0);
+  Config_PrintSettings(code_seen('S') && code_value() == 0);
 }
 
 #ifdef ABORT_ON_ENDSTOP_HIT_FEATURE_ENABLED
     SERIAL_ECHOLN(MSG_INVALID_EXTRUDER);
   }
   else {
-    boolean make_move = false;
+    #if EXTRUDERS > 1
+      bool make_move = false;
+    #endif
+
     if (code_seen('F')) {
-      make_move = true;
+      #if EXTRUDERS > 1
+        make_move = true;
+      #endif
       next_feedrate = code_value();
       if (next_feedrate > 0.0) feedrate = next_feedrate;
     }
   SERIAL_PROTOCOLLNPGM(MSG_OK);
 }
 
-void get_coordinates()
-{
-  bool seen[4]={false,false,false,false};
-  for(int8_t i=0; i < NUM_AXIS; i++) {
-    if(code_seen(axis_codes[i]))
-    {
-      destination[i] = (float)code_value() + (axis_relative_modes[i] || relative_mode)*current_position[i];
-      seen[i]=true;
+void get_coordinates() {
+  for (int i = 0; i < NUM_AXIS; i++) {
+    float dest;
+    if (code_seen(axis_codes[i])) {
+      dest = code_value();
+      if (axis_relative_modes[i] || relative_mode)
+        dest += current_position[i];
     }
-    else destination[i] = current_position[i]; //Are these else lines really needed?
+    else
+      dest = current_position[i];
+
+    destination[i] = dest;
   }
-  if(code_seen('F')) {
+  if (code_seen('F')) {
     next_feedrate = code_value();
-    if(next_feedrate > 0.0) feedrate = next_feedrate;
+    if (next_feedrate > 0.0) feedrate = next_feedrate;
   }
 }
 

Marlin/stepper.cpp

 }
 
 void microstep_init() {
-  const uint8_t microstep_modes[] = MICROSTEP_MODES;
-
   #if defined(E1_MS1_PIN) && E1_MS1_PIN >= 0
     pinMode(E1_MS1_PIN,OUTPUT);
     pinMode(E1_MS2_PIN,OUTPUT); 
     pinMode(Z_MS2_PIN,OUTPUT);
     pinMode(E0_MS1_PIN,OUTPUT);
     pinMode(E0_MS2_PIN,OUTPUT);
-    for (int i = 0; i <= 4; i++) microstep_mode(i, microstep_modes[i]);
+    const uint8_t microstep_modes[] = MICROSTEP_MODES;
+    for (int i = 0; i < sizeof(microstep_modes) / sizeof(microstep_modes[0]); i++)
+        microstep_mode(i, microstep_modes[i]);
   #endif
 }
 

Marlin/temperature.cpp

 
 int target_temperature[EXTRUDERS] = { 0 };
 int target_temperature_bed = 0;
-int current_temperature_raw[EXTRUDERS] = { 0 };
+int current_temperature_raw[4] = { 0 };
 float current_temperature[EXTRUDERS] = { 0.0 };
 int current_temperature_bed_raw = 0;
 float current_temperature_bed = 0.0;