thread model for mxe-octave build

[John W. Eaton]

I had someone ask why C++11 threads (mutext, condition_variable_any) 
wasn't working with .oct files built with Octave 4.2.1.  After a little 
searching, I found that we are configuring GCC with 
--enable-threads=win32 instead of --enable-threads=posix.  Apparently 
the latter is required to enable C++11 thread features.  Is there any 
reason to not use --enable-threads=posix by default in our builds?  I 
tried building native-gcc with this option and it allowed my test 
program to compile but I don't know whether there might be other 
problems.  From what I hve read, it seems that even if GCC is built with 
this option you can still use Windows threading features directly.  I've 
seen some mention of performance issues "in some situations" but no 
explanation of what that might be.

jwe