Per source build options

[Mikko Rapeli <mikko.rapeli%teleca.com@localhost>]

While trying to build userspace for arm/thumb instrunction set, I needed
a way to set per source build options. Since the bsd make manual is nice enough
to show a way, I also documented my slightly different version to
http://wiki.netbsd.se/Per_file_build_options incase someone else might
like to find this.

If you have easier, cleaner etc. ways of doing the same, I'd like to
hear about them. Or just update the wiki.

Thanks,

-Mikko