API: Make header just work with C++

It can now be included bare in C++ files, instead of wrapped
in an extern "C" block.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
API: Add dllimport handling

This is similar to how x264 and x265's API works.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
MSVC: Always build against static CRT

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
API: Use consistent parameter list format.