Made the teensy3/Makefile completely Arduino independent.

Those that specify a NO_ARDUINO environment variable before using
the teensy3/Makefile, will be completely Arduino independent. Note
however that if ARDUINOPATH is set, the NO_ARDUINO behaviour will
be overridden. With NO_ARDUINO set and ARDUINOPATH unset, the
cross-compiler tool path (COMPILERPATH) will be set to /usr/bin.
Setting the COMPILERPATH environment variable before using the
teensy3/Makefile, will override the default.

teensy3/Makefile

 # The name of your project (used to name the compiled .hex file)
 TARGET = main
 
+# Those that specify a NO_ARDUINO environment variable will
+# be able to use this Makefile with no Arduino dependency.
+# Please note that if ARDUINOPATH was set, it will override
+# the NO_ARDUINO behaviour.
+ifndef NO_ARDUINO
 # Path to your arduino installation
 ARDUINOPATH ?= ../../../../..
 #ARDUINOPATH ?= ../../../..
+endif
 
 # configurable options
 OPTIONS = -DF_CPU=48000000 -DUSB_SERIAL -DLAYOUT_US_ENGLISH -DUSING_MAKEFILE
 # locations and edit the pathnames.  The rest of Arduino is not needed.
 #************************************************************************
 
+ifdef ARDUINOPATH
+
 # path location for Teensy Loader, teensy_post_compile and teensy_reboot
 TOOLSPATH = $(abspath $(ARDUINOPATH)/hardware/tools)   # on Linux
 
 # path location for the arm-none-eabi compiler
 COMPILERPATH = $(abspath $(ARDUINOPATH)/hardware/tools/arm/bin)
 
+else
+# Default to the normal GNU/Linux compiler path if NO_ARDUINO
+# and ARDUINOPATH was not set.
+COMPILERPATH ?= /usr/bin
+
+endif
+
 #************************************************************************
 # Settings below this point usually do not need to be edited
 #************************************************************************
 %.hex: %.elf
 	$(SIZE) $<
 	$(OBJCOPY) -O ihex -R .eeprom $< $@
+ifneq (,$(wildcard $(TOOLSPATH)))
 	$(TOOLSPATH)/teensy_post_compile -file=$(basename $@) -path=$(shell pwd) -tools=$(TOOLSPATH)
 	-$(TOOLSPATH)/teensy_reboot
-
+endif
 
 # compiler generated dependency info
 -include $(OBJS:.o=.d)