Use the __COUNTER__ macro instead of __LINE__

[aedrax]

This fixes the issue where a compiler puts multiple case statements on the same line and then has a conflict between the two.

For example, for whatever reason, somebody wanted to put

async_yield; async_yield;

They wouldn't be able to because it would expand into

*_async_k = 5; return ASYNC_CONT; case 5:; *_async_k = 5; return ASYNC_CONT; case 5:;

but this will (effectively) expand into:

*_async_k = 3; return ASYNC_CONT; case 3:; *_async_k = 5; return ASYNC_CONT; case 5:;

For what it's worth, __COUNTER__ is not in the C standard but is in most compilers

[naasking]

Hmm, I like the idea but don't like that it's not standard. I created async.h for use on my microcontroller projects that come with their own C compilers. Then again, not really a big deal to run the gcc/clang preprocessor and then the C compiler on the preprocessed source, so I'll have to think about this...

[shakna-israel]

What about a fallback using something like...

#ifndef __COUNTER__
    #define __COUNTER__ __LINE__
#endif