Return Maybe Platform as part of compiler configure, and place it in LocalBuildInfo as hostPlatform.
GHC infers the platform form ghc --info using new 'platformFromTriple' function. Other compilers return Nothing, which triggers fallback to old behavior of using buildPlatform. hostPlatform is then threaded through to initialPathTemplateEnv.

Makes things a little simpler in GHC's build system if libraries are in
the same directory as their name.

At the top level we now have cabal/ and cabal-install/

The knownExtensions list had already got out of sync with the datatype.
Now we use an Enum instance to get the list of constructors.
Also, having the actual extensions in a separate type than UnknownExtension
and EnableExtension makes it easier to add extension disabling support
later.

New fields default-language and other-languages for specifying the
base languages for the package, e.g. Haskell98, Haskell2010
New fields default-extensions and other-extensions for the language
extensions. Separate from the old extensions field.
The separation lets us express the difference between declaring to
the outside world that a package uses certain languages or extensions
and whether certain languages or extensions should be applied to
all modules in the package component.

So the generic code iterates over the libs/exes rather than
the per-compiler code doing that. It'll help when we have
more complex inter-component dependencies.
This is also the proper fix for ticket #525.