Completed the patch by adding the rounding-control bits to the mask (_MCW_RC) and making sure the macros only get defined for MS-VC compiler (#ifdef _MSC_VER).

Ran the tests (python_d -m test.autotest) on win32. Seems OK. The tests that were skipped or failed don't seem to be connected to this patch:
- test_repr failed on trying to import a very long package and module name
- test_popen failed with SyntaxError: unexpected EOF while parsing.

Here's the summary:
323 tests OK.
2 tests failed:
   test_popen test_repr
2 tests altered the execution environment:
   test_distutils test_site
62 tests skipped:
   test_aepack test_al test_applesingle test_bsddb test_bsddb185
   test_bsddb3 test_bz2 test_cd test_cl test_codecmaps_cn
   test_codecmaps_hk test_codecmaps_jp test_codecmaps_kr
   test_codecmaps_tw test_commands test_crypt test_curses test_dbm
   test_dl test_epoll test_fcntl test_fork1 test_gdb test_gdbm
   test_gl test_grp test_imgfile test_ioctl test_kqueue
   test_largefile test_linuxaudiodev test_macos test_macostools
   test_mhlib test_nis test_openpty test_ossaudiodev test_pipes
   test_poll test_posix test_pty test_pwd test_py3kwarn test_readline
   test_resource test_scriptpackages test_smtpnet test_socketserver
   test_sqlite test_ssl test_sunaudiodev test_tcl test_threadsignals
   test_timeout test_tk test_ttk_guionly test_ttk_textonly
   test_urllib2net test_urllibnet test_wait3 test_wait4
   test_zipfile64
10 skips unexpected on win32:
   test_bsddb test_bz2 test_gdb test_readline test_sqlite test_ssl
   test_tcl test_tk test_ttk_guionly test_ttk_textonly
[43048 refs]