Stop include Bullet headers using `-isystem` for GCC/Clang as it misleads
SCons into not properly rebuilding all files when headers change.
This means we also need to make sure Bullet builds without warning, and
current version fares fairly well, there were just a couple to fix (patch
included).
Increase minimum version for distro packages to 2.90 (this was never released
as the "next" version after 2.89 was 3.05... but that covers it too).
b7901c773c