Support for network-2.6
Merge branch '1.18' of https://github.com/snoyberg/cabal into 1.18
Build C sources with -fPIC when GHC is using dynamic libraries.

Fixes #2207.
Bump Cabal version number to 1.18.1.5
The download dir on haskell.org moved
Bump cabal-install version number to 1.18.0.6
Also build profiling C objs with '-fPIC' when GHC is dynamic.
Fix test suite compilation with network 2.6