Another attempt at fixing htools build without curl

OK, my previous small fix was not good. There is another issue: haddoc
(the documentation generator) needs to pass the same compiler options
(i.e. in our case, -DNO_CURL) to ghc. But in case of no curl, then it
shouldn't scan at all the RAPI library, as that is not used in our
builds.

Clearly, this is not a nice thing. So this patch changes from
including/excluding RAPI conditionally (in two places, the
ExtLoader.hs module and in hscan.hs), to always include RAPI, and
moves the curl/no curl login to RAPI itself, where it belongs.

Together with passing --optghc to haddock, this makes the builds
consistent both with and without RAPI. I also undo the removal of RAPI
from QC.hs.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>