cabal configure with build-type: Configure should append rather than prepend --configure-options to the ./configure script

`cabal configure` with `build-type: Configure` should append rather than prepend the `--configure-option`s to the `./configure` script. As it is, there's no way to override the flags that Cabal supplies on its own, such as `--with-gcc`. Current behavior:

```
./configure [configure-options] --with-compiler=… --prefix=… --with-gcc=…
```

Preferred behavior:

```
./configure --with-compiler=… --prefix=… --with-gcc=… [configure-options]
```