Fix i386/windows build.

With the new -XNegativeLiterals, the compiler was (rightly) complaining
that values like (0x80000000 :: Int32) suffer from signed overflow. This
caused the Windows GHC build to fail, since it uses -Werror.

The fix is very simple. For details, see Note [Overflow checking and
fromIntegral] in Graphics/Win32/GDI/HDC.hs

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Bump version to 2.3.0.1 (re #13)

Note: this version bump does not yet represent a package release, but has
been only done so we can ship GHC 7.8 RC1 with a `win32` package version
distinguishable from what shipped with GHC 7.6.3.

Also I bumped the min-version build-dep on `base` as the recent changes
to the `win32` code-base rely on features only available in `base-4.7.0.0`.