Build: fix 775 configure should detect doc reqs. (issue2019041)

[percival . music . ca]

Reviewers: ,

Message:
Ok, hearing no complaints... and since this doesn't add any build
requirements (it just checks if you have the existing reqs, instead of
letting you attempt to make doc and then die halfway through), I pushed
it.

Description:
Doc: CG: update requirements for doc build.


Build: fix 775 configure should detect doc reqs.

It's a trivial patch, but I feel a bit sheepish proposing any
changes to the build system after making such a big deal about a
"build system freeze" back in June and July.  :|

Please review this at http://codereview.appspot.com/2019041/

Affected files:
  M Documentation/contributor/introduction.itexi
  M Documentation/contributor/source-code.itexi
  M Documentation/included/compile.itexi
  M Documentation/included/helpus.itexi