First pass at trying to standardize httpd-2.0's usage of APR and APR-util

so that they can be separated - use an installed APR and APR-util with
httpd-2.0.  This is not expected to achieve full separation, but is
merely the first step in the process.  The tree should still be buildable
in the source-only case.

This commit does the following things:
- Use the standard find_apr.m4 and find_apu.m4 m4 macro files that flood and
  Subversion already use for locating APR and APR-util.  This adds the
  --with-apr and --with-apr-util configure options.
- Stop using APRVARS and export_vars.sh.
- Use apr-config and apu-config to determine dependencies.
- Add AP_BUILD_SRCLIB_DIRS and AP_CLEAN_SRCLIB_DIRS values (names stink)
  to dynamically figure out what subdirs in srclib we will build.
  (What we really want to do is set one value - go L->R on build and
   R->L on clean.  I can't think of a way to do this.)
- If we are using APR and APR-util that are already installed, don't
  specify them in srclib.  (This stil...