bug 1855: libmudflap -- MinGW thread

To address bug 1855 [1], I try to add --enable-libmudflap to build gcc.

It failed as:

...
checking for thread model used by GCC... win32
win32 is an unsupported thread package
make[1]: *** [configure-target-libmudflap] Error 1
...

The direct cause from configure.ac:

...
# We only support posix threads, or no threads at all.
posix_threads=
case ${target_thread_file} in
   posix)
     posix_threads=yes
     ;;
   single)
     ;;
   *)
     echo "${target_thread_file} is an unsupported thread package" 1>&2
     exit 1
     ;;
esac
...

I believe MinGW has a POSIX interface for threading (pthreads-w32?). Is 
this true? If yes what is your suggestion on patching this. Adding 
'win32)\n posix_threads=yes' directly there looks ugly.

You have a good weekend!

[1] https://sourceforge.net/p/mingw/bugs/1855/

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