Unclarity of configure options

The current (CVS version) configure script has the following options
(among many others):
 --enable-dtrace         build with DTrace support --with-ossp-uuid        build with OSSP UUID library for UUID
generation--with-libxml           build with XML support --with-libxslt          build with XSLT support
 

One could think that adding any of this option to ./configure before
building Postgres from sources, he will have corresponding support
after installation and initdb process. But what we have now is the
huge difference between "--with-libxml" and "--with-libxslt": while
the first one adds XML support to the core, the second one doesn't
provide anything automatically, it allows only using contirb/xml2
(what is unclear because the help message is the same as for
--with-libxml -- "build with ... support").

Also, comparing --enable-dtrace and --with-libxml I cannot see any
difference in its semantics: --enable-dtrace also depends on external
library and configure process fails if the system doesn't have it. So
why "--enable-" is used in the first case and "--with-" in the second
one?

-- 
Best regards,
Nikolay