patches to warn when JAVA_HOME not set and therefore build may fail

[Barrie Treloar <Ba...@camtech.com.au>]

We seem to be getting a lot of people trying to build ant and not
reading the build.sh file to see what the problem might be when it
can't find sun.tools.* classes.

This patch only affects the Unix files as I dont know how to make the
changes for windows.

All it does it print a warning if JAVA_HOME is not set, I dont know of
a better way to do this other than using grep to see if tools.jar or
classes.zip is already in the classpath.

I patched both bootstrap.sh and ant.

I've also updated the README file to explicitly list the build and
install links of the documentation so people know what steps are
needed to build ant.  

I've updated index.html to include setting the JAVA_HOME variable for
the build process and added a link to the installing process to show
how this is done for your operating system.

The only caveat with this patch is that people are unlikely to read
the warning output :) but at least its there.

Barrie
--
Barrie Treloar
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  Barrie Treloar                      Phone: +61 8 8303 3300
  Senior Analyst/Programmer           Fax:   +61 8 8303 4403 
  Electronic Commerce Division        Email: barrie@camtech.com.au
  Camtech (SA) Pty Ltd                http://www.camtech.com.au
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