Makefile should allow a user editable dependency directories

[jacobrosenthal]

In testing I've found I need to bring in a few other libraries. I'm currently doing this like so

DEP_DIR = ../../ringbuffer

# Flags passed to the preprocessor.
# Set Google Test's header directory as a system directory, such that
# the compiler doesn't generate warnings in Google Test headers.
CPPFLAGS += -isystem $(GTEST_DIR)/include -isystem $(GMOCK_DIR)/include \
        -I$(ARDUINO_MOCK_DIR)/include/arduino-mock/ \
        -I$(DEP_DIR)

but it only allows for a single include. If you're amenable some kind of bash iterator should allow a list of directories to be added