For the configure newbies among us:
If you update or reinstall any of the libraries used to compile in a different directory than they started out, you will need to make sure that you update the config.cache file (or re-generate it) so that configure will not look in the wrong place for the information.
On Mac OS X, for example, I updated my libxml using Fink. Fink placed the files in the /sw directory. However, php was still looking for important libxml files (such as xml2-config) in the old directory (/usr/bin/xml2-config). After updating config.cache with the new value of the xml2-config path, I was able to compile correctly.