hadrian: Fix building from source-dist without alex/happy

This fixes two bugs which were adding dependencies on alex/happy when
building from a source dist.

* When we try to pass `--with-alex` and `--with-happy` to cabal when
  configuring but the builders are not set. This is fixed by making them
  optional.
* When we configure, cabal requires alex/happy because of the
  build-tool-depends fields. These are now made optional with a cabal
  flag (build-tool-depends) for compiler/hpc-bin/genprimopcode.

Fixes #21627