Pass make options to Particle cloud compile

blink.ino

// Build with
// particle compile photon . build.mk
void setup() {
  pinMode(BLINK_PIN, OUTPUT);
}

void loop() {
  digitalWrite(BLINK_PIN, HIGH);
  delay(1000);
  digitalWrite(BLINK_PIN, LOW);
  delay(1000);
}

build.mk

# See USER_MAKEFILE in https://github.com/particle-iot/firmware/blob/develop/docs/build.md#variables
CPPFLAGS+=-DBLINK_PIN=D7