Arduino due BOSSAC source code build using the source tree makefile

Question

I have downloaded the bossa source code from - https://github.com/shumatech/BOSSA/tree/arduino

I installed Cygwin for windows and built the source code from the Cygwin shell by executing the following make command:

make OS=MINGW32_NT-6.1

I get the following error:

In file included from src/WinPortFactory.h:33:0,
             from src/WinPortFactory.cpp:29:
src/PortFactory.h:59:2: error: #error "Platform is not supported"
 #error "Platform is not supported"
  ^
Makefile:180: recipe for target 'obj/WinPortFactory.o' failed
 make: *** [obj/WinPortFactory.o] Error 1

After inspecting the PortFactory.h file, I see a pre processor directive __WIN32__.

I have tried to include that in my build as follows and get no success:

make OS=MINGW32_NT-6.1 CPPFLAGS=-D__WIN32__

What am I missing? I have CFLAGS and CXXFLAGS as well and get no success.