It seems to no longer do anything useful, and it also prevents
CI from detecting that include/bytestring-cpp-macros.h
was not getting picked up by `cabal sdist`.

In particular, this ensures that `cabal sdist` grabs it.

The cabal documentation claims as a pre-requisite for its
usability "when compiling the package itself."  It's unclear
if this is really true, but there is no harm done in adding
bytestring-cpp-macros.h there just in case.

It seems this macro is never visible in Haskell code.
But with real unaligned primops in the near future,
it doesn't seem worth messing with.