default configuration parameters

[Massimo Manghi <ma...@unipr.it>]

Default values of configuration parameters are hard coded in mod_rivet and the 
docs don't say for each of them what actually is the default value.

I took up the problem and decided to define preprocessor symbols for them that 
could be controlled through 'configure' switch. I preserved the defaults I 
found in mod_rivet.c, the difference is they now are written in configure.ac 
instead.

I left out of the process the configurable scripts. I think it would be easy to 
somehow extend the mechanism for those which are meant to be run at global 
level (global init/exit or child init/exit), whereas it's impossible for 
scripts that pertain to specific directories or are user defined as they rely 
on Apache's internal mechanics.
 
This setup should allow the build of nearly preconfigured mod_rivet modules and 
it might come handy for certain installations.

I wait for objections before committing. Docs will be updated only if the 
patches are approved.

-- Massimo