http: Reduce scope of a variable in parse_content_encoding()

Also fixes an unused variable warning with zlib disabled.
w32pthreads: Mark functions in compatibility wrapper as av_unused

This avoids annoying warnings about unused functions. The compatibility
wrapper is designed to provide a complete (stub) API, so some functions
being unused by some files is natural and no reason for a warning.
atomic_win32: Drop unnecessary atomic.h #include

The file does not require atomic.h. Also kills function redefinition warnings.
vfwcap: Replace deprecated av_destruct_packet() by av_free_packet()