Fix an error when EXTRUDE_MINTEMP is not defined

Marlin/Conditionals.h

@@ -27,10 +27,21 @@
 
 #ifndef CONDITIONALS_H
 
+/**
+* Miscellaneous
+*/
 #ifndef M_PI
   #define M_PI 3.1415926536
 #endif
 
+/**
+ * This value is used by M109 when tying to calculate a ballpark safe margin
+ * to prevent wait-forever situation.
+ */
+#ifndef EXTRUDE_MINTEMP
+ #define EXTRUDE_MINTEMP 170
+#endif
+
 #ifndef CONFIGURATION_LCD // Get the LCD defines which are needed first
 
   #define PIN_EXISTS(PN) (defined(PN##_PIN) && PN##_PIN >= 0)