Build:
    - On Windows (MSVC), the _USE_MATH_DEFINES macro is no longer defined when
    including <openvdb/Platform.h> (or any dependent headers). If you were
    relying on this in your own project for M_PI, M_PI_2, etc. you can add
    -D_USE_MATH_DEFINES to your own project compiler options. See
    https://docs.microsoft.com/en-us/cpp/c-runtime-library/math-constants
    for more info.

Add release note for _USE_MATH_DEFINES change
Add release note for _USE_MATH_DEFINES change

Signed-off-by: Edward Lam <15e8b15f4bf4dca51ba7bbf117ec97e9d35017f8@yahoo.com>
