add feature detection machinery

include/ring-buffer-config.h

@@ -0,0 +1,22 @@
+#ifndef _RING_BUFFER_CONFIG_H
+#define _RING_BUFFER_CONFIG_H
+
+#ifdef RING_BUFFER_NOEXCEPT
+#define NOEXCEPT(x) noexcept(x)
+#else
+#define NOEXCEPT(x)
+#endif
+
+#ifdef RING_BUFFER_CONSTEXPR
+#define CONSTEXPR constexpr
+#else
+#define CONSTEXPR
+#endif
+
+#ifdef RING_BUFFER_CONSTEXPR_DESTRUCTORS
+#define CONSTEXPR_D CONSTEXPR
+#else
+#define CONSTEXPR_D
+#endif
+
+#endif