This way people hopefully won't complain if these APIs
change and break code that used an older API.

Spot candidates by running these commands:
  git ls-files |xargs perl -0777 -n \
    -e 'while (/\b(then?|[iao]n|i[fst]|but|f?or|at|and|[dt]o)\s+\1\b/gims)' \
    -e '{$n=($` =~ tr/\n/\n/ + 1); ($v=$&)=~s/\n/\\n/g; print "$ARGV:$n:$v\n"}'

Thanks to Jim Meyering for the original patch.

This is the simplest method to do threading, which splits
the uncompressed data into blocks and compresses them
independently from each other. There's room for improvement
especially to reduce the memory usage, but nevertheless,
this is a good start.

This should have been in 5eefc008.

It didn't mention the return value that is used if
an error occurs.

This is cleaner and makes it simpler to add new members
to lzma_action enumeration.

This is based lzma_chunk_size() that was included in some
development version of liblzma.

Empty Block was created if the input buffer was empty.
Empty Block wastes a few bytes of space, but more importantly
it triggers a bug in XZ Utils 5.0.1 and older when trying
to decompress such a file. 5.0.1 and older consider such
files to be corrupt. I thought that no encoder creates empty
Blocks when releasing 5.0.2 but I was wrong.

This return value was missing from the API comments of
four functions.

The biggest problem was that the integrity check type
wasn't validated, and e.g. lzma_easy_buffer_encode()
would create a corrupt .xz Stream if given an unsupported
Check ID. Luckily applications don't usually try to use
an unsupport Check ID, so this bug is unlikely to cause
many real-world problems.

It's an internal function and it's not needed by
anything outside stream_encoder.c.

This reverts commit 352ac82d.
I don't know what I was thinking.

This adds:

  - mythread_sync() macro to create synchronized blocks

  - mythread_cond structure and related functions
    and macros for condition variables with timed
    waiting using a relative timeout

  - mythread_create() to create a thread with all
    signals blocked

Some of these wouldn't need to be inline functions,
but I'll keep them this way for now for simplicity.

For timed waiting on a condition variable, librt is
now required on some systems to use clock_gettime().
configure.ac was updated to handle this.

It's an internal function and it's not needed by
anything outside stream_encoder.c.

This is incompatible with the 8.3 support patch made by
Juan Manuel Guerrero. I think this one is nicer, but
I need to get feedback from DOS users before saying
that this is the final version of 8.3 filename support.

Try to avoid overwriting the source file if --force is
used and the generated destination filename refers to
the source file. This can happen with 8.3 filenames where
extra characters are ignored.

If the generated output file refers to a special file
like "con" or "prn", refuse to write to it even if --force
is used.

Passing --disable-decoders to configure broke a few
encoders due to missing #ifdefs in filter_common.c.

Thanks to Jason Gorski for the patch.

Now it always defaults to one thread. Maybe this
will change again if a threading method is added
that doesn't affect memory usage.

It was renamed to ax_pthread.m4 in Autoconf Archive.

It leaks old filter options structures (hundred bytes or so)
every time the lzma_stream is reinitialized. With the xz tool,
this happens when compressing multiple files.