Experimental build of libtink.so on Linux and Darwin, guarded by an option.

Add targets to build a tarball with headers included like in Bazel, using "make package" or equivalent. This is the very first take and brings CMake up to speed with Bazel. We'll improve on this in the next releases.

PiperOrigin-RevId: 240534917
GitOrigin-RevId: c5bcb02a1e1be8d3c95e9426a17f950e4511c352