Update XZ Utils to 5.0.3 (in deps)

TODO

 
 XZ Utils To-Do List
 ===================
 
 Known bugs
 ----------
 
     The test suite is too incomplete.
 
     If the memory usage limit is less than about 13 MiB, xz is unable to
     automatically scale down the compression settings enough even though
     it would be  possible by switching from BT2/BT3/BT4 match finder to
     HC3/HC4.
 
     The code to detect number of CPU cores doesn't count hyperthreading
     as multiple cores. In context of xz, it probably should.
     Hyperthreading is good at least with p7zip.
 
     XZ Utils compress some files significantly worse than LZMA Utils.
     This is due to faster compression presets used by XZ Utils, and
-    can be worked around by using "xz --extreme". However, the presets
-    need some tweaking and maybe this issue can be minimized without
-    making the typical case too much slower.
+    can often be worked around by using "xz --extreme". With some files
+    --extreme isn't enough though: it's most likely with files that
+    compress extremely well, so going from compression ratio of 0.003
+    to 0.004 means big relative increase in the compressed file size.
 
     xz doesn't quote unprintable characters when it displays file names
     given on the command line.
 
     tuklib_exit() doesn't block signals => EINTR is possible.
 
+    SIGTSTP is not handled. If xz is stopped, the estimated remaining
+    time and calculated (de)compression speed won't make sense in the
+    progress indicator (xz --verbose).
+
 
 Missing features
 ----------------
 
     xz doesn't support copying extended attributes, access control
     lists etc. from source to target file.
 
     Multithreaded compression
 
     Multithreaded decompression
 
     Buffer-to-buffer coding could use less RAM (especially when
     decompressing LZMA1 or LZMA2).
 
-    I/O library is not implemented. It will possibly be named libzzf.
+    I/O library is not implemented (similar to gzopen() in zlib).
+    It will be a separate library that supports uncompressed, .gz,
+    .bz2, .lzma, and .xz files.
 
     lzma_strerror() to convert lzma_ret to human readable form?
     This is tricky, because the same error codes are used with
-    slightly different meanings.
+    slightly different meanings, and this cannot be fixed anymore.
 
 
 Documentation
 -------------
 
     Some tutorial is needed for liblzma. I have planned to write some
     extremely well commented example programs, which would work as
     a tutorial. I suppose the Doxygen tags are quite OK as a quick
     reference once one is familiar with the liblzma API.
 
     Document the LZMA1 and LZMA2 algorithms.