thread/Posix{Mutex,Cond}: disable "constexpr" on Android

Bionic's pthread declarations are non-literal.

src/thread/PosixCond.hxx

@@ -41,7 +41,10 @@ class PosixCond {
 	pthread_cond_t cond;
 
 public:
-	constexpr PosixCond():cond(PTHREAD_COND_INITIALIZER) {}
+#ifndef __BIONIC__
+	constexpr
+#endif
+	PosixCond():cond(PTHREAD_COND_INITIALIZER) {}
 
 	PosixCond(const PosixCond &other) = delete;
 	PosixCond &operator=(const PosixCond &other) = delete;

src/thread/PosixMutex.hxx

@@ -41,7 +41,10 @@ class PosixMutex {
 	pthread_mutex_t mutex;
 
 public:
-	constexpr PosixMutex():mutex(PTHREAD_MUTEX_INITIALIZER) {}
+#ifndef __BIONIC__
+	constexpr
+#endif
+	PosixMutex():mutex(PTHREAD_MUTEX_INITIALIZER) {}
 
 	PosixMutex(const PosixMutex &other) = delete;
 	PosixMutex &operator=(const PosixMutex &other) = delete;