Patch up some flaws, here and there

Marlin/Conditionals_post.h

     #ifndef LCD_FEEDBACK_FREQUENCY_DURATION_MS
       #define LCD_FEEDBACK_FREQUENCY_DURATION_MS 100
     #endif
-  #elif PIN_EXISTS(BEEPER)
+  #else
     #ifndef LCD_FEEDBACK_FREQUENCY_HZ
       #define LCD_FEEDBACK_FREQUENCY_HZ 5000
     #endif
     #ifndef LCD_FEEDBACK_FREQUENCY_DURATION_MS
       #define LCD_FEEDBACK_FREQUENCY_DURATION_MS 2
     #endif
-  #else
-    #ifndef LCD_FEEDBACK_FREQUENCY_DURATION_MS
-      #define LCD_FEEDBACK_FREQUENCY_DURATION_MS 2
-    #endif
   #endif
 
   /**

Marlin/Marlin.h

 
 #define SERIAL_ECHOPAIR(name,value) (serial_echopair_P(PSTR(name),(value)))
 
+void serial_echopair_P(const char* s_P, const char *v);
 void serial_echopair_P(const char* s_P, char v);
 void serial_echopair_P(const char* s_P, int v);
 void serial_echopair_P(const char* s_P, long v);

Marlin/Marlin_main.cpp

   void plan_cubic_move(const float offset[4]);
 #endif
 
+void serial_echopair_P(const char* s_P, const char *v)   { serialprintPGM(s_P); SERIAL_ECHO(v); }
 void serial_echopair_P(const char* s_P, char v)          { serialprintPGM(s_P); SERIAL_CHAR(v); }
 void serial_echopair_P(const char* s_P, int v)           { serialprintPGM(s_P); SERIAL_ECHO(v); }
 void serial_echopair_P(const char* s_P, long v)          { serialprintPGM(s_P); SERIAL_ECHO(v); }

Marlin/SanityCheck.h

 #endif
 
 /**
+ * Delta requirements
+ */
+#if ENABLED(DELTA)
+  #if DISABLED(USE_XMAX_PLUG) && DISABLED(USE_YMAX_PLUG) && DISABLED(USE_ZMAX_PLUG)
+    #error "You probably want to use Max Endstops for DELTA!"
+  #endif
+#endif
+
+/**
  * Babystepping
  */
 #if ENABLED(BABYSTEPPING)

Marlin/ultralcd.cpp

       lcd.buzz(duration, freq);
     #elif PIN_EXISTS(BEEPER)
       buzzer.tone(duration, freq);
+    #else
+      UNUSED(duration); UNUSED(freq);
     #endif
   }