The C SDK is designed for portability and can be ported to most any OS, RTOS, or even simple taskers. So, basically if you set your makefile correctly for your OS and use right libraries for it, it should work.
In your Makefile you must specify what OS you will be using. You do this by defining TW_OS_INCLUDEto point to the required include file for your OS, as shown here for an example to run on Linux:
You *should* be able to compile it for NT without a problem. If there are compilation problems they will almost certainly be in src/porting/twWindows.c or /src/porting/twWin32Threads.c.
You will run into some issues with IRIX assuming you are running on old SGI hardware with a MIPS processor. But if you start using the OSX build (gcc-osx-x86-64) files in build/platform that should take care of POSIX & BSD sockets issues leaving you with having to only tweak the compiler flags. If you are doing a native compile that may not be a big deal, might only need to change the CC_OPTS and LIBOPTS flags. You also may run into endian issues (the default build is for little endian) but if you put -DBIG_ENDIAN_CPU in your compiler options, that should take care of that.
Feel free to contact me directly if you need some help.