Clean up CPP/Haddock interaction

This removes the obsolete pre-Haddock2 `__HADDOCK__` conditional check
which isn't supported anymore by Haddock2 (and in fact the conditional
isn't needed anyway).

This commit also moves more content into the
`System/FilePath/Internal.hs` template that's `#include`d by
`System/FilePath/{Posix,Windows}.hs` and as a side-effect, a part of the
Haddock module-level documentation containing usage examples is now
shown again in the generated HTML output.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Refactor & update `filepath.cabal` to `cabal-version>=1.10`

As of this commit, `filepath` compiles cleanly with
GHC 6.10/6.12/7.0/7.2/7.4/7.6/HEAD.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Add .travis.yml to test compilation with older GHCs

This is currently a proof of concept and might be disabled again.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>