[PATCH 0 of 3] improve checking for documentation tools and formatting

[Matt Wilson <msw@amazon.com>]

This patch series is a follow up to a change that I posed for the last
Xen Documentation Day that used lynx to create plain text from
markdown files. I've added ./configure time checking for all the tools
used in the docs/ tree. The user can persistently override one of
these tools at ./configure time by setting the appropriate environment
variable. The docs/ tree maintains the list of default tools, so
running ./configure is not a prerequisite for running "make -C docs".

I've switched to using elinks instead of lynx for creating text
documentation, as it provides better formatting and interface compared
to other tools. Of course the user can override the tool and flags if
they'd like to use something else.

Here's a sample of ./configure output after this series is applied:

checking for ps2pdf... /usr/bin/ps2pdf
checking for dvips... no
configure: WARNING: dvips is not available so some documentation won't be built
checking for latex... no
configure: WARNING: latex is not available so some documentation won't be built

Please let me know if there are other concerns that need to be
addressed.

Matt