util/RefCount: no "constexpr" with libc++

Not supported by libc++.

NEWS

@@ -2,6 +2,7 @@ ver 0.18.4 (not yet released)
 * decoder
   - dsdiff: fix byte order bug
 * fix build failures due to missing includes
+* libc++ compatibility
 
 ver 0.18.3 (2013/11/08)
 * fix stuck MPD after song change (0.18.2 regression)

src/util/RefCount.hxx

@@ -42,7 +42,12 @@ class RefCount {
 	std::atomic_uint n;
 
 public:
-	constexpr RefCount():n(1) {}
+#ifndef _LIBCPP_VERSION
+	/* the "constexpr" is missing in libc++'s "atomic"
+	   implementation */
+	constexpr
+#endif
+	RefCount():n(1) {}
 
 	void Increment() {
 		++n;