Removed CPP extension

This is primarily to support Mac users who have problems with the CPP extension
due to Clang.

This means that there is no longer a Haskell98 mode for `pipes` (sorry
@archblob).  Eventually, CPP support will go back to normal on Mac and then I
will add back the Haskell98 mode for `pipes`, but for now it will be temporarily
disabled until that time comes.

The other thing this effects is that `pipes` no longer compiles against
`mtl < 2.1.0`.  However, these are very old versions of `mtl`.  To give an idea
of how old these versions are, Debian Stable packages `mtl-2.1.1`.  These old
versions could also be supported by also removing the offending methods from the
`mtl` type class instances, but that seems unnecessary (again, because it's so
rare to encounter such old `mtl` versions).

Finally, the `Trustworthy` pragmas are no longer guarded.  Even Debian Stable
packages `ghc-7.4`, which supports this extension, so there's no point guarding
these behind CPP macros any longer.