thread/Posix{Cond,Mutex}: don't ues PTHREAD_*_INITIALIZER on NetBSD

On NetBSD, PTHREAD_MUTEX_INITIALIZER and PTHREAD_COND_INITIALIZER are
not compatible with C++11 "constexpr" (see Mantis ticket 0004110).  As
a workaround, don't ues "constexpr", and use the functions
pthread_mutex_init(), pthread_mutex_destroy(), pthread_cond_init() and
pthread_cond_destroy() instead.  This adds some runtime overhead, but
is portable to POSIX implementations that have awkward initializer
macros.

src/thread/PosixCond.hxx

@@ -41,7 +41,21 @@ class PosixCond {
 	pthread_cond_t cond;
 
 public:
+#ifdef __NetBSD__
+	/* NetBSD's PTHREAD_COND_INITIALIZER is not compatible with
+	   "constexpr" */
+	PosixCond() {
+		pthread_cond_init(&cond, nullptr);
+	}
+
+	~PosixCond() {
+		pthread_cond_destroy(&cond);
+	}
+#else
+	/* optimized constexpr constructor for sane POSIX
+	   implementations */
 	constexpr PosixCond():cond(PTHREAD_COND_INITIALIZER) {}
+#endif
 
 	PosixCond(const PosixCond &other) = delete;
 	PosixCond &operator=(const PosixCond &other) = delete;

src/thread/PosixMutex.hxx

@@ -41,7 +41,21 @@ class PosixMutex {
 	pthread_mutex_t mutex;
 
 public:
+#ifdef __NetBSD__
+	/* NetBSD's PTHREAD_MUTEX_INITIALIZER is not compatible with
+	   "constexpr" */
+	PosixMutex() {
+		pthread_mutex_init(&mutex, nullptr);
+	}
+
+	~PosixMutex() {
+		pthread_mutex_destroy(&mutex);
+	}
+#else
+	/* optimized constexpr constructor for sane POSIX
+	   implementations */
 	constexpr PosixMutex():mutex(PTHREAD_MUTEX_INITIALIZER) {}
+#endif
 
 	PosixMutex(const PosixMutex &other) = delete;
 	PosixMutex &operator=(const PosixMutex &other) = delete;