fix compile when using clang as $CC:

Based on patch from https://bugs.launchpad.net/hugin/+bug/1213585 (rebased)

--- a/include/zthread/Guard.h
+++ b/include/zthread/Guard.h
@@ -108,7 +108,7 @@
   }
 
   template <class LockType>
-  static void createScope(LockHolder<LockType>& l, unsigned long ms) {
+  static bool createScope(LockHolder<LockType>& l, unsigned long ms) {
 
     if(Scope1::createScope(l, ms))
       if(!Scope2::createScope(l, ms)) {