It seems that calling `lldb` when it is absent triggers some popup
proposing to install Xcode on macOS. This is obviously not good. Let's
check presence with g_find_program_in_path() instead. I was refraining
from doing so until now, because this function allocates memory, hence
may not do well during a crash.
Fortunately we don't need to check for lldb during crash (unlike gdb
which has some unacceptable behavior for older versions, at least on
FreeBSD) so that should be ok.
(cherry picked from commit 04bbe941d5)
3bbebaf6e0