thread/{Cond,Mutex}: use "class" instead of "typedef"

Allows forward-declaration.

src/thread/Cond.hxx

@@ -25,12 +25,12 @@
 /* mingw-w64 4.6.3 lacks a std::cond implementation */
 
 #include "WindowsCond.hxx"
-typedef WindowsCond Cond;
+class Cond : public WindowsCond {};
 
 #else
 
 #include "PosixCond.hxx"
-typedef PosixCond Cond;
+class Cond : public PosixCond {};
 
 #endif
 

src/thread/Mutex.hxx

@@ -25,13 +25,12 @@
 /* mingw-w64 4.6.3 lacks a std::mutex implementation */
 
 #include "CriticalSection.hxx"
-typedef CriticalSection Mutex;
+class Mutex : public CriticalSection {};
 
 #else
 
 #include "PosixMutex.hxx"
-
-typedef PosixMutex Mutex;
+class Mutex : public PosixMutex {};
 
 #endif