Add Azteeg X3 Pro as motherboard 68

Add digipot i2c control for MCP4451
Allow M907 to set i2c digipot currents in amps
Fix Makefile to allow Azteeg motherboards
Fix Makefile to allow Wire libraries only
Add beeper pin for Azteeg X3 Pro

Marlin/Marlin.ino

@@ -3,17 +3,17 @@
 /*
     Reprap firmware based on Sprinter and grbl.
  Copyright (C) 2011 Camiel Gubbels / Erik van der Zalm
- 
+
  This program is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.
- 
+
  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
- 
+
  You should have received a copy of the GNU General Public License
  along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
@@ -22,8 +22,8 @@
  This firmware is a mashup between Sprinter and grbl.
   (https://github.com/kliment/Sprinter)
   (https://github.com/simen/grbl/tree)
- 
- It has preliminary support for Matthew Roberts advance algorithm 
+
+ It has preliminary support for Matthew Roberts advance algorithm
     http://reprap.org/pipermail/reprap-dev/2011-May/003323.html
  */
 
@@ -50,3 +50,7 @@
 #if defined(DIGIPOTSS_PIN) && DIGIPOTSS_PIN > -1
 #include <SPI.h>
 #endif
+
+#if defined(DIGIPOT_I2C)
+  #include <Wire.h>
+#endif