system-source: Fix the uname() call on Solaris

On Solaris, uname() can return any positive value, and that indicates
success. Pretty much everywhere else, only 0 does. Thankfully, -1
signals failure everywhere, so check for that.

This makes system() work on Solaris again.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>