Add support for the Digilent chipKIT platform

This commit modifies a few things in Arduino.mk to be able to support
overriding the necessary configuration options to support the chipKIT build,
and also includes an example sketch configured to build for the Max32 platform.

In addition the following changes were done as well

- Control C standard with a CFLAGS_STD flag, GNU99 by default.
- Duplicate show_config_variable in each sub-makefile.
- Remove redundant output of ARDUINO_PREFERENCES_PATH from config table.
- Defer Priting ARDMK_DIR until Arduino.mk.
- Define names of compiler tools earlier to use for path checking.

Duplicating show_config_variable is unfortunate because the code is duplicated,
but I don't see a good way around it since we need to dupliacate the check
for ARDMK_DIR before we can find Common.mk.