Windows: Define TUKLIB_SYMBOL_PREFIX in config.h.

It is to keep all symbols in the lzma_ namespace.
Windows: Update the build script and README-Windows.txt.

The 32-bit build is now for i686 or newer because the
prebuilt MinGW-w64 toolchains include i686 code in the
executables even if one uses -march=i486.

The build script builds 32-bit SSE2 enabled version too.
Run-time detection of SSE2 support would be nice (on any OS)
but it's not implemented in XZ Utils yet.
Windows: Update build instructions.
Update INSTALL about the dependencies of the scripts.
xz: Fix a comment.
Update NEWS for 5.0.8.
Update NEWS for 5.2.0.
po/fr: missing line in translation of --extreme.
po/fr: improve wording for help for --lzma1/--lzma2.
Fix build when --disable-threads is used.
Build: Include 04_compress_easy_mt.c in the tarball.
Avoid variable-length arrays in the debug programs.
Bump version and soname for 5.2.0.

I know that soname != app version, but I skip AGE=1
in -version-info to make the soname match the liblzma
version anyway. It doesn't hurt anything as long as
it doesn't conflict with library versioning rules.
Docs: Update docs/examples/00_README.txt.
Docs: Use lzma_cputhreads() in 04_compress_easy_mt.c.
Windows: Fix bin_i486 to bin_i686 in build.bash.
DOS: Update Makefile.