intentional bootstrap requirements

[nia <nia%NetBSD.org@localhost>]

ideas:

- explicitly document that c99 is needed for bootstrap,
  c11 or more recent versions should not be used in
  packages that are in the bootstrap chain

- remove gcc2 and references to pre-c99 compilers,
  as well as ability to specify dependencies for
  pre-c99 compilers.

would also like a rule on _maximum_ language versions, but
that's more controversial and a separate discussion.