the stack is non-executable if not explicitely enabled.
The latter one is not enabled in the Makefile yet because it cannot
succeed on platforms where the non-exec stack is not implemented.
Both checks rely on the fact that gcc puts trampoline onto the stack
for nested functions. On platforms where no such trampolines are used,
no meaningful results can be expected.
While not perfect, this is much easier than hand-coding trampolines for
each CPU type.
