On OSX don't use atexit() to destroy context.
On OSX don't use atexit() to destroy context.

Unlike on Linux, on OSX destructor registered by pthread_key_create
seems to be run for the main thread.

Signed-off-by: Martin Sustrik <4dd6061be1198639e8b05ce4fd5ead7a0dcaa0f4@250bpm.com>

sustrik/libdill,sustrik/libdill,sustrik/libdill,sustrik/libdill