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.