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.