First hack at supporting standalone packages.

What this means is that a non-GHC fptools project which sets
$(PACKAGE) in a Makefile should do something reasonable.  It works by
testing whether $(ProjectNameShort) == "ghc", and if it isn't, then:

 - it turns off all the machinery that deals with installing the
   package for use with the in-place compiler and a (future)
   installation of GHC.

 - instead, it enables a rule that installs the package using
   the installed $(GHC_PKG).  The configure script still needs
   updating to detect this, so 'make install' won't work yet.

 - $(GHC) is used instead of $(GHC_INPLACE) for compiling Haskell
   code.

Apart from the detection of ghc-pkg in configure, this should be all
that's needed to get source distributions and 'make install' going.