Link system tsk (statically), talloc (dynamically) instead of embedding
Note to future maintainers (or myself): This patch needs updating every once in a while as upstream's setup.py is changed.
Upstream's general logic seems to be to have Sleuthkit sources unpacked into a subdirectory sleuthkit/, run configure without any extras, and build selected C or C++ files from sleuthkit/tsk as well as the C or C++ files in the pytsk root directory. Also, talloc is built from source.
We instead link libtsk statically (bot everything else dynamically), by passing "-Wl,-Bstatic -ltsk -Wl,-Bdynamic" to the linker.
We want to dynamically link libtalloc as found on the system as well as some libtsk dependencies, currently: libafflib, libewf, libvhdi, libvmdk, libsqlite3, libz. (cf. output of "objdump -x libtsk.so | grep NEEDED").
Apparently, libtsk dependencies need to be passed as extra_link_args after -ltsk.
