Update MQTTAsync.c

Merge branch 'bugfix-double-free-for-qos0' of https://github.com/locez/paho.mqtt.c into locez-bugfix-double-free-for-qos0

remove '-MD' in ADD_DEFINITIONS command. why add the -MD?
this -MD forcible control MSVC generate code with /MD option
if user want to compile with /MT by set option -DCMAKE_USER_MAKE_RULES_OVERRIDE=compiler_flag_overrides.cmake in command line,
the '-MD' definition cause it not work;