Add support for autodetecting the mac's alternate platforms dir

build/arduino-mk/Arduino.mk

@@ -335,7 +335,10 @@ ifndef ARDUINO_SKETCHBOOK
     ifneq ($(ARDUINO_SKETCHBOOK),)
         $(call show_config_variable,ARDUINO_SKETCHBOOK,[AUTODETECTED],(from arduino preferences file))
     else
-        ARDUINO_SKETCHBOOK = $(HOME)/sketchbook
+
+         ARDUINO_SKETCHBOOK := $(firstword \
+            $(call dir_if_exists,$(HOME)/sketchbook) \
+            $(call dir_if_exists,$(HOME)/Documents/Arduino/) )
         $(call show_config_variable,ARDUINO_SKETCHBOOK,[DEFAULT])
     endif
 else