add --with-dbm switch to configure, allowing the user to set the type of DBM

add --with-dbm switch to configure, allowing the user to set the type of DBM
  that apu_dbm will use (defaults to builtin sdbm). first pass at gdbm
  config support (seems to work fine in my simple test).

- added src/lib/aputil/apu_private.h.in to hold aputil config info
- apu_dbm.c now uses #if for the config values (rather than #ifdef)
- cleared out temp hack in configure.in to force sdbm usage

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87100 13f79535-47bb-0310-9956-ffa450edef68