Rework the podspec

Switch to building from source to better match how CocoaPods wants to work and
reduce the required download size, and add a subspec with only the headers to
work around the issues with overlinking the libraries in unit tests.

Closes #690.

@alazier 
I propose something like this instead:

```
if [ -L core]; then
   target=$(readlink core) || exit 1
   unlink core || exit 1
   mkdir -p core
   cp -R target/* core/* || exit 1
else
   exit 1
fi
```

This would allow not to deal with `/tmp`, and the risk of overloading `/tmp/core` in case of concurrent builds (for CI for example)