Add properly gated extern "C" to public headers

[tedbrandston]

I'd like to add this so that consumers don't need to worry about it.

From #161 I gather that a PR that does this would be welcome. I think it should be sufficient to add something like the following to each of the headers:

#ifdef __cplusplus
extern "C" {
#endif

Just logging my intent to open a PR soon.

[tedbrandston]

Waiting on some local testing, but current code is here: tedbrandston@2190005

I'm leaving /* __cplusplus */ comments off of the endifs, because it looks like those are only used for endifs far from their condition.

So far I've tested that make and make test work.