Add support for foreign libraries.

A stanza for a platform library looks something like

    platform-library test-package
      type:                native-shared

      if os(Windows)
        options: standalone
        mod-def-file: TestPackage.def

      other-modules:       MyPlatformLib.Hello
                           MyPlatformLib.SomeBindings
      build-depends:       base >=4.7 && <4.9
      hs-source-dirs:      src
      c-sources:           csrc/MyPlatformLibWrapper.c
      default-language:    Haskell2010

where native-shared means that we want to build a native shared library
(.so on Linux, .dylib on OSX, .dll on Windows). The parser also
recognizes native-static but this is not currently supported anywhere.
The standalone option means that the we merge all library dependencies
into the dynamic library (i.e., ghc options -shared -static), rather
than make the created dynamic library just record its dependencies (ghc
options -shared -dynamic); it is currently compulsory on Windows and
unsupported anywhere else. The mod-def-file can be used to specify a
module definition file, and is also Windows specific.

There is a bit of refactoring in Build: gbuild is the old buildOrReplExe
and now deals with both executables and platform libraries.