Mingw32CCompiler in cygwincompiler.py emits the symbol -mno-cygwin.
This is used to make Cygwin's gcc behave as mingw. As of gcc 4.6 it is not recognized by the mingw gcc compiler itself, and causes as crash. It should be removed because it is never needed for mingw (in any version), only for cross-compilation to mingw from other gcc versions.
Instead, those who use CygwinCCompiler or Linux GCC to "cross-compile" to plain Win32 can set -mno-cygwin manually. It also means -mcygwin should be removed from the output of CygwinCCompiler.