fix for MinGW

This patch makes it possible to compile pigz on MinGW for Windows.
* Tested with the latest published compiler from the mingw project (4.7.2)
* Disabled a few warnings when compiled on mingw, which sprung up when I nulled the functions that are missing on Windows.
* Also made it explicit that the c99 mode should be used during compilation, as I take it that was your implicit intent. Without setting it explicitly, mingw's GCC fails to find the enhanced printf-function that is able to handle the extended formats you use.