Some people reported cases where gdb doesn't even seem to understand
Python scripts (the error in #12904 implies it is looking for standard
gdb commands, because apparently -x option allows both styles).
Others had issues in sandbox building (#12956).
I don't really want to spend too much time understanding each case,
especially as it's a self-debugging tool, not absolutely necessary for a
successful compilation.
So let's just make this optional, and default to "false".
c3bc4e5336