Fix Raspbian Buster build for Pi 4

[jdonald]

These are my minimal changes to get things compiling and installing on Buster. Note I also required one outstanding PR on mupen64plus-audio-sdl.

Unless running on a Pi A/A+/B+/0/0w, it's too easy to trip up on build.sh and install.sh pointing to different default RaspbianLists if someone just glosses that part over in the README. Rather than copypaste the selector conditions into install.sh and dev_build.sh I added a safeguard around this in build.sh.

High-level goal is not to break the build flow for Jessie as trunk seems to be designed for, although users will now have to explicitly specify GCC=4.7 for that target.

Testing on a Pi 4: I left the video-gles2n64 plugin settings unchanged to avoid breaking existing behavior, but as this plugin is compiled for legacy VideoCore (VC=1) it's only valid for a Pi <=3B+ in legacy mode. You can test instead with the Rice plugin that doesn't care:

mupen64plus --gfx mupen64plus-video-rice.so 'Super Mario 64 (U) [!].z64'

Or if you wish to test video-gles2n64 on the Mesa driver, remove VC=1 on the relevant line in RaspbianList_Pi2.