Add support for building without curl

Since curl is not always needed (e.g. when only using luxi or less
likely file backends only) and is also not always available, it is
useful for building without it. This of course disabled the RAPI
backend.

This patch changes ExtLoader to build with the ‘-cpp’ option which makes
ghc run it through cpp first; and based on whether ‘NO_CURL’ is defined
or not, this toggles RAPI/curl inclusion. The patch also removes the
import of Rapi in QC.hs since it's not actually used in tests.

Invoking make as ‘make HEXTRA=-DNO_CURL’ is enough to trigger the new
build mode.