Add better checks and docs for build tools

Add better checks and docs for build tools

Prompted by a report from Ed Avis:
<http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/14710>

* README-hacking: Organise LZMA and Valgrind as
as optional requirements rather than in their own sections.
Mention bootstrap will now check tool versions.
* README-prereq: Make a start on specific instructions
for optaining build tools. Currently we just have notes
for Fedora linux.
* bootstrap.conf: Add the list of tools and versions required.
* bootstrap: Add the logic to check for the required tools,
and list all required tools and versions if any are missing.