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).
This updates our local copy to commit 5ec8339b6fc491e3f09a34a4516e82787f053fcc.
We need a recent master commit for some new features that we use in Godot
(see #25543 and #28909).
To avoid warnings generated by Bullet headers included in our own module,
we include those headers with -isystem on GCC and Clang.
Fixes#29503.
