Fix CMAKE_HOST_SYSTEM_NAME on SunOS

In commit 0bbd993f (Make CMAKE_HOST_SYSTEM_NAME available in scripting
context, 2016-12-26) we added a call to `uname` that checks for a zero
return value.  However, on Solaris the `uname(2)` manual [1] says that
on success a non-negative value is returned.  Fix our return code check
so that we detect the `SunOS` name correctly.

[1] https://docs.oracle.com/cd/E53394_01/html/E54765/uname-2.html