I just got a Bela Mini and have compiled some PD externals for it and thought I would contribute a more verbose guide for others. I hope this is helpful and that people can help show me some other ways of achieving this so I can learn too.
Download PD-Extended Source Code. We will use this to transfer to the Bela and then build externals on the board. I downloaded the source from here: https://puredata.info/downloads/pd-extended
Unzip the package. This ended up in the Downloads folder for me.
Open a terminal. CD to the externals folder of PD-Extended
> cd Downloads/
> cd pd-extended/
Copy the externals folder to the Bela's pd-externals folder in the projects directory. Make sure you have it plugged in and have a working network connection first. You may have to create the pd-externals folder first by SSHing into the Bela.
> scp -r externals/ firstname.lastname@example.org:/root/Bela/projects/pd-externals/
SSH into the Bela.
> ssh email@example.com
Move to the externals folder.
> cd Bela/projects/pd-externals/externals
Move to the folder of your choice and run the build files. Mileage will vary. bsaylor and sigpack are easy and sound great.
> cd bsaylor
> make PD_INCLUDE=/usr/local/include/libpd/
Copy the resulting .pd_linux binaries into the pd-externals folder.
> cp *pd_linux /root/Bela/projects/pd-externals/
Load a PD project with the externals in it and enjoy.
Thanks for posting this, so helpful!