Fix build issues on CentOS

Fix build issues on CentOS

Hi.  I had problems compiling modwaklog using CentOS and the latest
openafs from git.

I managed to work out generally-useful solutions to the problems, so I
thought I'd send this patch on.

1. I added a configuration option to tell it where to find the apr
   headers.

2. The afs headers to include have changed.  It was complaining that
   afs_int32 was undefined.  I now include <afs/param.h>, which is
   apparently the accepted thing to do for afs programs.

3. When my build was completed, I found the .so file not in the main
   directory, but in .libs/mod_waklog.so.  I updated the documentation to
   say to look for it there.

I attached a patch that fixed these problems for me.

Thanks.
--Ryan