Define "machdep" attribute and mark files (in amd64 and x86).
Use option dependency to always select KERN (and its dependencies).
regen gcc mknative files for new complex and more.
bump libstdc++ minor due to new complex etc functions enabled.
If an attribute is selected, select its dependencies too.
If an option is selected, select the matching (lowercased) attribute.  Handle
dependencies too.
Now that attribute dependency works, prefer it to options dependency (personal
preference).  Use it for "kern", depending on "machdep" and "uvm".
Select attributes for filesystem options too.
- Support DH89XXCC devices.
- Add extra delay for 82580 and newer devices except DH89XXCC SGMII device.
  Same as FreeBSD.
Allow dependency on undefined attributes, so that attribute definitions can
be written out-of-order, like:

	# b is not defined yet
	define	a: b
	file	a.c	a

	# b is defined here
	define	b
	fine	b.c
Two more files missed in last addition
Put TODO ideas (to keep your fun).
Document -M.
Fix two typos.