Add extern to global vars

[darbyjohnston]

Doing some searching it seems that extern and extern "C" are not the same; the first declares that something has external linkage, while the second declares that the symbols are not mangled in C++. This PR hopefully fixes #22 by adding extern to the global variables, along with extern "C" when compiling with C++.

To use extern with extern "C" I needed to add curly brackets, so I created a couple of new macros for defining a block of statements as extern "C".

[meshula]

Oh, a legendary hero has arrived!!! Thanks!

That's a pattern I hadn't thought of at all. I wonder if this would work, also

#ifdef __cplusplus
#define OTIO_EXTERN extern "C" extern
#else
#define OTIO_EXTERN extern
#endif

and then OTIO_EXTERN below, instead of OTIO_API in those cases.

The only reason for the alternate pattern being that one wouldn't need the scoping braces, so the macro could be used anywhere.

If you want to try this, great, otherwise, I'm ready to land as is. LMK which you prefer.

[darbyjohnston]

That works with MSVC but not with GCC:

error: invalid use of ‘extern’ in linkage specification
   31 | #define OTIO_EXTERN extern "C" extern

I'm not super happy with the macros I added but couldn't think of a better way to add the curly brackets. Though one positive is you can now replace multiple uses of OTIO_API with a pair of OTIO_API_BEGIN and OTIO_API_END.

[meshula]

Ok, thanks for checking!