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

Source/cmStateSnapshot.cxx

@@ -308,7 +308,7 @@ void cmStateSnapshot::SetDefaultDefinitions()
   this->SetDefinition("CMAKE_HOST_UNIX", "1");
 
   struct utsname uts_name;
-  if (uname(&uts_name) == 0) {
+  if (uname(&uts_name) >= 0) {
     this->SetDefinition("CMAKE_HOST_SYSTEM_NAME", uts_name.sysname);
   }
 #endif