Debian packages to install before building Plan9Port

This is as much for my own benefit as anyone else because I forget which ones I need every single time.

On Debian 10, I installed the following packages before compiling Plan 9 Port:

• build-essential
• libx11-dev
• libxcb1-dev
• x11proto-core-dev
• libxt-dev
• libfontconfig1-dev
• libxext-dev