Fix build of cbits/HsOpenSSL.c when using a different build dir.

The build fails when using a different build directory than 'dist':

$ cabal configure --builddir=dist12
$ cabal build --builddir=dist123

In file included from cbits/HsOpenSSL.c:1:0:

cbits/HsOpenSSL.h:30:50:
     fatal error: ../dist/build/autogen/cabal_macros.h: No such file or directory
compilation terminated.