This fixes trac #2266.

-fglasgow-exts also turns it on.

We no longer create those directories

If it has been built then we pass it, even if we are still using the
bootstrapping compiler.

These are needed for GLOBAL_VAR's to work properly

It's probably to work around a long-dead bug

...which is good, as the conditional test was broken anyway!

I assume that we still want this, despite it having been disconnected
when the module was moved.

fixes crash with -threaded -debug for me

We shouldn't need it, as we don't call cygwin's gcc, and it was causing
problems with the nightly builders passing it to GHC.

So now

  :load foo.hs       loads bytecode for foo.hs, even if foo.o exists
  :load foo          is just shorthand for :load foo.hs
  :load M            loads a module M, as object code if possible
                     (no change here)

  :set -fobject-code
  :load foo.hs       loads foo.hs as object code; an existing foo.o
                     can be used.
  
This turned out to be very straightforward: when building the
ModSummary for a file (summariseFile) we just ignore the object file
unless -fobject-code is on.