Makefile: Support newer versions of cabal that use the new-build system.

Makefile: Support newer versions of cabal that use the new-build system.

Unfortunately, cabal puts the binary in a very complicated path
and does not provide any good way to get it out, leaving no good choice
except to use find.

It may be possible to use cabal (new-)install --symlink-bindir,
and ask it to symlink to pwd, but with my older version of cabal,
that does not work.

The stack branch will probably also break once it uses a newer cabal,
didn't try to deal with that.