dist-xz, dist-bzip2: don't hard-code -9: honor envvar settings

dist-xz, dist-bzip2: don't hard-code -9: honor envvar settings

* lib/am/distdir.am (dist-xz): Do not hard-code xz's -9: that
made it impossible to override.  Instead, use its XZ_OPT envvar,
defaulting to -9 if not defined.  Thus no change in behavior
when XZ_OPT is not set, and now, this rule honors the setting
of that envvar when it is set.  Suggested by Lasse Collin.
(dist-bzip2): Likewise for it's corresponding envvar: BZIP2.
* NEWS (Miscellaneous changes): Mention it.
* doc/automake.texi (The Types of Distributions): Describe the
newly enabled environment variables.