Add custom var to define the web_prefixdir.

* I like to put my ganglia-web install in a non-standard location so I added
  custom_web_prefixdir to the spec so all you need to do is pass a define to
  rpmbuild such as:
  rpmbuild -bb --define 'custom_web_prefixdir /var/www/ganglia' ganglia-web.spec

ganglia-web.spec.in

@@ -12,6 +12,8 @@ Obsoletes: ganglia-webfrontend
 Requires: php >= 5, php-gd
 %if 0%{?suse_version}
 %define web_prefixdir /srv/www/htdocs/ganglia
+%elseif %{?custom_web_prefixdir}
+%define web_prefixdir %{custom_web_prefixdir}
 %else
 %define web_prefixdir /var/www/html/ganglia
 %endif