Control C standard with a CFLAGS_STD flag, GNU99 by default.
Duplicate show_config_variable in each sub-makefile.

This 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.
Remove redundant output of ARDUINO_PREFERENCES_PATH from config table.