$OpenBSD$

OpenBSD doesn't have malloc_{,usable_}size()

--- src/zreaders/m_alloc.cpp.orig	Sun Jan  3 13:19:00 2016
+++ src/zreaders/m_alloc.cpp	Sun Jan  3 13:20:47 2016
@@ -57,7 +57,7 @@ namespace GC
 #endif
 #if defined(__APPLE__)
 #define _msize(p)				malloc_size(p)
-#elif defined(__sun)
+#elif defined(__sun) || defined(__OpenBSD__)
 #define _msize(p)				(*((size_t*)(p)-1))
 #elif !defined(_WIN32)
 #define _msize(p)				malloc_usable_size(p)	// from glibc/FreeBSD