Extend --with-zstd documentation

Extend --with-zstd documentation

The patch that was so far added for documenting --with-zstd is pretty
minimal:
  - it refers to undocumented options --with-zstd-include and
    --with-zstd-lib;
  - it suggests that --with-zstd can be used without an argument;
  - it does not clarify how this option applies to cross-compilation.

How about adding the same details as for the --with-isl,
--with-isl-include, --with-isl-lib options, mutatis mutandis? This patch
does that.

	PR other/105527

gcc/ChangeLog:

	* doc/install.texi (Configuration): Add more details about --with-zstd.
	Document --with-zstd-include and --with-zstd-lib

Signed-off-by: Bruno Haible <bruno@clisp.org>