cabal new-build --disable-shared doesn't disable shared libraries.

Looking at the `ProjectPlanning.hs`, I believe `--disable-shared` is only applied to the root package? And not propagated recursively? While this is exactly what I want for `--enable-shared` in #4617, it is exactly not what I want for `--disable-shared`.

I want `--disable-shared`, because `yesod-persistent` with `new-build` blows past the mach-o load command size:
```
malformed mach-o: load commands size (33576) > 32768
```
on macOS Sierra.

The underlying reason I believe is that we embed
```
@rpath/libHS---ghc.dylib
```
for each dependent library.