In mingw32, int16_t is not defined by sys/types.h, but it is by stdint.h, and it is in the int16_t man page as being defined in stdint.h. Thanks to mithi for help debugging.
Copied sources from http://beesbuzz.biz/code/audiocompress/AudioCompress-2.0.tar.gz [mk: created this patch under fluffy's name and fixed some gcc signed/unsigned comparison warnings]