Make pak compatible with R installations w/o a shared library

[gaborcsardi]

On Linux. On Windows and macOS we can assume that there is a shared library.

There are two ways to do this:

• Remove the shared link to libR from all package shared libs with patchelf when building the pak binary. Seemingly it is not needed and the packages work fine without it. Hard to know if this is always true, though.
• Add a dummy libR.so file somewhere where the linker finds it. We could set LD_LIBRARY_PATH for the subprocess to facilitate this. We need to create a shared lib that works on all platforms, though. Again, hard to know if this always works.

[gaborcsardi]

The dummy libR.so is hard because we cannot change LD_LIBRARY_PATH for the R process itself, and we also need to load some shared objects in the main process. So we need to put the libR.so in a place that is on the library search path already. But that is not really possible to manage from an R package. So we need to remove the libR.so dependency with patchelf at build time.