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.
Update TODO.
liblzma: Document the need for block->check for lzma_block_header_decode().

Thanks to Tomer Chachamu.
Update THANKS.
Fix typos in comments.
xz: Fix use of wrong variable.

Since the only call to suffix_set() uses optarg
as the argument, fixing this bug doesn't change
the behavior of the program.
liblzma: Remove a useless C99ism from sha256.c.

Unsurprisingly it makes no difference in compiled output.
liblzma: Avoid C99 compound literal arrays.

MSVC 2013 doesn't like them. Maybe they aren't so good
for readability either since many aren't used to them.
xz: Fix a comment.
xz: Rename a variable to avoid a namespace collision on Solaris.

I don't know the details but I have an impression that there's
no problem in practice if using GCC since people have built xz
with GCC (without patching xz), but renaming the variable cannot
hurt either.

Thanks to Mark Ashley.
Update INSTALL.

Add a note about failing "make check". The source of
the problem should be fixed in libtool (if it really is
a libtool bug and not mine) but I'm unable to spend time
on that for now. Thanks to Nelson H. F. Beebe for reporting
the issue.

Add a note about a possible need to run "ldconfig" after
"make install".
Build: Add --disable-doc to configure.
Build: Fix the combination of --disable-xzdec --enable-lzmadec.

In this case "make install" could fail if the man page directory
didn't already exist at the destination. If it did exist, a
dangling symlink was created there. Now the link is omitted
instead. This isn't the best fix but it's better than the old
behavior.