[PATCH] Allow _MSC_EXTENSIONS macro on non-Windows targets

[Yunzhong Gao]

  Hi David,
  Many thanks for the code sample! I can reproduce the behavior with clang and with Visual Studio.
  I still have a question regarding the MSVC behavior. I used Visual Studio 2012 and when I change
  the project property to disable language extensions (/Za), Visual Studio will leave the static const
  member as undefined. This seems to indicate that if clang wants to match MSVC behavior, it
  should only produce a definition at -fms-extensions mode rather than being on by default, unless
  there is a way to disable this behavior. Maybe I am still missing something.
  - Gao

http://llvm-reviews.chandlerc.com/D3034