thread/Id: drop "::" prefix before pthread function names

The "::" to explicitly refer to the global namespace appeared like a
good idea in C++, but it breaks with C libraries that implement
standard functions using macros (e.g. musl).

NEWS

@@ -1,4 +1,5 @@
 ver 0.18.20 (not yet released)
+* fix build failure with musl
 
 ver 0.18.19 (2014/11/26)
 * archive

src/thread/Id.hxx

@@ -75,7 +75,7 @@ public:
 #ifdef WIN32
 		return ::GetCurrentThreadId();
 #else
-		return ::pthread_self();
+		return pthread_self();
 #endif
 	}
 
@@ -84,7 +84,7 @@ public:
 #ifdef WIN32
 		return id == other.id;
 #else
-		return ::pthread_equal(id, other.id);
+		return pthread_equal(id, other.id);
 #endif
 	}