I was faced with the prospect of rebuilding the PHP interpreter on a server, to add support for GD.
Googling for info yielded this article with some good overview of the process.
After I downloaded the sources and started the configure process I was hit with a slew of errors related to the fact that there were missing some dependencies. Stuff like "xml2-config not found. Please check your libxml2 installation."
One cure for that seems to install the development version of those components, like so:
yum install component-devel
For me googling the error messages pointed me, in order, to the following resources that helped me solve the issues, here, here, here and finally here.