Add -Op option for msvc compiler.

The option is "Improve float consistency" and without it, we get failure
of C++ Boost's numeric/convertion/udt_example_0 test.


[SVN r33183]

tools/build

@@ -1 +1 @@
-Subproject commit a79ef83bd434735ab346969e6cd1df77eebf4ad8
+Subproject commit 6375efa5a66f5dd5a09b111f5aa69d9cd8947a66