This is primarily intended for use with the Cabal test suite (allowing
us to easily specify multiple Cabal packages for the same Haskell source
files), but maybe some end-users will find it useful as well.  If there
are multiple Cabal files in the current working directory, --cabal-file
(for configure) allows you to disambiguate which one to build with.

There's a big hack to handle the BOM check, as it is inconvenient to
plumb the flag value all the way to the check code.  Some bigger
refactoring needed, see #3552.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>