Clarify wording for some subtyping errors
The wording of a small number of errors relating to the subtyping of capabilities were improved.
These were previously technically incorrect, or otherwise unnecessarily obtuse.
Fix inability to fully build pony on Raspberry PI 4's with 64-bit Raspbian
Building on 64-bit Raspbian needs the flag -fPIC instead of -fpic which is used by default on Unix builds.
We've added a Makefile flag pic_flag that you can set to either -fpic or -fPIC for both libs and configure steps, e.g. make libs pic_flag=-fPIC and make configure pic_flag=-fPIC. It will default to -fpic if not specified.
Added build instructions for 64-bit Raspbian
We've gotten Pony building on 64-bit Raspbian running on Raspberry Pi 4 model B boards. The installation is a little different than other Linux distributions that you can build Pony on, so we've added instructions specific to 64-bit Raspbian.