Require libzstd and remove zlib support

Require libzstd and remove zlib support

* zlib has been removed. Good riddance!
* libzstd is now required for building ccache. However, it’s not bundled
  like zlib was.
* To make it easier to build ccache on systems that lack an easily
  installable libzstd, the configure script now offers a
  --with-libzstd-from-internet option, which downloads a zstd source
  release archive, unpacks it in the tree and sets up the Makefile to
  build the library and link ccache (statically) with it.
* Enabled compression by default.
* Made compression level 0 mean “use a default level suitable for the
  current compression algorithm”. For zstd, that’s initially level -1,
  but that could change in the future. The reason for using 0 as a
  special marker is that a future alternative compression algorithm
  could have another reasonable default than zstd. (Let’s hope that
  future algorithms don’t use level 0 for something.)
* Changed default compression level to 0.

Closes #413.