I recently had to upgrade Subversion on an RHEL 4 machine; Subversion 1.1.4 was installed and that's getting a little long in the tooth. So I downloaded and tried to compile Subversion, but ran into problems because the RHEL 4 version of APR is pretty old too. Specifically, I got this error:

checking APR version... 0.9.4
wanted regexes are 0\.9\.[7-9] 0\.9\.1[0-9] 1\.
configure: error: invalid apr version found

So I poked around for a bit and figured out how the --with-apr and --with-apr-util options work. This works out well, actually, since I've got an upgraded version of Apache and now I can point Subversion to it. For posterity, here's the invocation:

./configure  \
  --with-ssl \
  --enable-shared \
  --without-berkeley-db \
  --with-apxs=/usr/local/apache2/bin/apxs \ 
  --with-apr=/usr/local/apache2/bin/apr-1-config \
  --with-apr-util=/usr/local/apache2/bin/apu-1-config

Hope this helps someone!