They are very required. The configure script will fail outright if
either of those libraries are absent.
libjpeg (or libjpeg-turbo) is a relatively new requirement, added as
of 0.9.9, but libpng has been a requirement since around 0.3.0.
> I'm trying to optimize my guacamole-server with the libjpeg62-turbo-dev
> package, but I don't see any reference to it at all in the build process and
> I'm trying to verify that it's being incorporated.
The libraries present will be summarized by the configure script if
the minimum build requirements are met. If you have libjpeg (or
libjpeg-turbo), you will see libjpeg listed as present in the summary.