On Linux, when /proc/sys/kernel/yama/ptrace_scope is 1, a process
may only ptrace its descendants by default, which prevents the GDB
process spawned by gimp_stack_trace_print() from attaching to, and
producing a backtrace for, the calling process.

Use prctl() with PR_SET_PTRACER, when available, in the parent
process, to allow the child process to ptrace it.