Avoid valgrind warnings in mod_ssl random number generator

We intentionally add uninitialized stack memory. To avoid warnings,
make valgrind believe that the memory is defined.

Add configure option to enable valgrind support


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1442307 13f79535-47bb-0310-9956-ffa450edef68