msvs: WinRT support

msvs: WinRT support

To compile x264 for WinRT the following additional steps has to be performed.

 * Ensure that the necessary SDK is installed.

 * Set the correct environment variables in the VS command prompt as shown at
   https://trac.ffmpeg.org/wiki/CompilationGuide/WinRT

 * Add one of the following to --extra-cflags depending on the target OS:
   "-DWINAPI_FAMILY=WINAPI_FAMILY_PC_APP -D_WIN32_WINNT=0x0A00" (Windows 10)
   "-DWINAPI_FAMILY=WINAPI_FAMILY_PC_APP -D_WIN32_WINNT=0x0603" (Windows 8.1)