[CSS2] Building CSS2

[frivoal]

@bert-github

For the css2 staging spec (/css2), this builds the spec:

cd csswg-drafts/css2
make

This works just fine on my machine. For the stable version (/css21), that should work as well, but there are a few differences in the Makefile, and it doesn't run on my machine.

1. Are the differences in the Makefile intentional / necessary?
2. Can the Makefile from /css21 be fixed? If it is not broken, can its dependencies be documented?

[gsnedders]

This has effectively been fixed, though we should document it better.