cabal repl can fail with C sources not linked -fPIC

Seen running `cabal repl` in git-annex:

relocation R_X86_64_PC32 against undefined symbol `statvfs@@GLIBC_2.2.5' can not be used when making a shared object; recompile with -fPIC

I suppose this is because the relevant C source file is included in an Executable stanza in my cabal file. Since it's not built as a library, cabal doesn't build it -fPIC.

Workaround: cabal configure --enable-executable-dynamic

```
cabal-install version 1.22.0.1
using version 1.22.1.1 of the Cabal library
The Glorious Glasgow Haskell Compilation System, version 7.8.4
```