Add support for relative lookup of template-hsc.h (#2)

If the `IN_GHC_TREE` CPP macro is defined (controlled by the `in-ghc-tree` cabal flag),
as a last resort, we also try to locate the template relative to the location of the currently
executed `hsc2hs`. Note that this is a hack to work around
only partial relocatable support in cabal, and is here to allow the `hsc2hs`
built and shipped with ghc to be relocatable with the ghc
binary distribution it ships with.

Note that on Windows, `getExecutablePath` doesn't yet operate correctly
in the presence of symlinks; but this should note have any detrimental
effects. This will be fixed/improved in future `base` version.