link gfortran on freebsd

Without this patch, I get this error when linking an executable:

```
Linking dist/dist-sandbox-d0be673a/build/findmelike/findmelike ...
/usr/local/bin/ld: /usr/home/vagrant/findmelike/.cabal-sandbox/lib/x86_64-freebsd-ghc-7.8.3/hmatrix-0.16.0.6/libHShmatrix-0.16.0.6.a(vector-aux.o): undefined reference to symbol 'cpow@@GFORTRAN_C99_1.0'
//usr/local/lib/gcc48/libgfortran.so.3: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
```

Unfortunately it is very difficult to produce a minimal test case that triggers this. I have only tested this on FreeBSD 10.0.