hadrian: Make it easier to debug Cabal configure

Right now, output is squashed. This make per-package configure scripts
extremely hard to maintain, because we get vague "library is missing"
errors when the actually probably is usually completely unrelated except
for also involving the C/C++ toolchain.

(I can always pass `-VVV` to Hadrian locally, but these errors are
subtle and I often cannot reproduce them locally!)

`--disable-option-checking` was added back in
75c6e068 but seems to be a bit overkill;
if other flags are passed that are not recognized behind the two from
Cabal mentioned in the former comment, we *do* want to know about it.