use system-wide libtalloc with --enable-system-talloc

This introduces a new configure flag by which the libosmocore-internal
talloc code is not compiled, but rather a system-wide libtalloc is used.

When we started openbsc/libosmocore in 2008, libtalloc was not widely
present on systems yet.  This has changed meanwhile, and we should
simply use the system-wide library
4 files changed