Build: Create liblzma.pc in a src/liblzma/Makefile.am.

Previously it was done in configure, but doing that goes
against the Autoconf manual. Autoconf requires that it is
possible to override e.g. prefix after running configure
and that doesn't work correctly if liblzma.pc is created
by configure.

A potential downside of this change is that now e.g.
libdir in liblzma.pc is a standalone string instead of
being defined via ${prefix}, so if one overrides prefix
when running pkg-config the libdir won't get the new value.
I don't know if this matters in practice.

Thanks to Vincent Torri.
Build: Remove a comment about Automake 1.10 from configure.ac.

The previous commit supports silent rules and that requires
Automake 1.11.
Add native threading support on Windows.

Now liblzma only uses "mythread" functions and types
which are defined in mythread.h matching the desired
threading method.

Before Windows Vista, there is no direct equivalent to
pthread condition variables. Since this package doesn't
use pthread_cond_broadcast(), pre-Vista threading can
still be kept quite simple. The pre-Vista code doesn't
use anything that wasn't already available in Windows 95,
so the binaries should run even on Windows 95 if someone
happens to care.
xz: Add a missing test for TUKLIB_DOSLIKE.
liblzma: Add block_buffer_encoder.h into Makefile.inc.

This should have been in b465da5988dd59ad98fda10c2e4ea13d0b9c73bc.
liblzma: Add LZMA_FULL_BARRIER support to single-threaded encoder.

In the single-threaded encoder LZMA_FULL_BARRIER is simply
an alias for LZMA_FULL_FLUSH.
liblzma: Support LZMA_FULL_FLUSH and _BARRIER in threaded encoder.

Now --block-list=SIZES works with in the threaded mode too,
although the performance is still bad due to the use of
LZMA_FULL_FLUSH instead of the new LZMA_FULL_BARRIER.
xz: Take advantage of LZMA_FULL_BARRIER with --block-list.

Now if --block-list is used in threaded mode, the encoder
won't need to flush at each Block boundary specified via
--block-list. This improves performance a lot, making
threading helpful with --block-list.

The flush timer was reset after LZMA_FULL_FLUSH but since
LZMA_FULL_BARRIER doesn't flush, resetting the timer is
no longer done.
xz: Document --flush-timeout=TIMEOUT on the man page.