Improve directory structure for in-tree iOS development

While developing and testing, it's often useful to link against libraries in dist (as opposed to linking with a framework). This spares the time needed to build and install the framework, and allows a simple cycle of, e.g.:
1. `cd source/ios_arm64/gst-plugins-bad-1.0-static-1.5/gst/foobar`
2. (fix bug in gstfoobar.c)
3. `make install`
4. Run your iOS tester in Xcode (triggers re-linking)

iOS requires static libraries, and we're apparently moving them to `lib/gstreamer-1.0/static` in `recipes/custom.py`. However, the recipe code doesn't get invoked on `make install`.

I think it'll be better if recipes/custom.py would **symlink** instead. That way, lib/gstreamer-1.0/static could still be used as the library path in Xcode, while `make install` would also work.

Also, I think `dist//lib/static` should be created along the same lines -- by symlinking `../*.a` into it.

Inspect recipes/custom.py and the existing dist//lib and lib/gstreamer-1.0/static layouts first. Check how make install currently handles these paths and verify the proposed symlinks let Xcode use the static libraries while preserving the edit, install, and relink cycle for the iOS tester.

Written by the indexing model from the issue text.