fix build failure on arm:
implicit declaration of function '_Offsetof' is invalid in C99 
Index: include/apr_general.h
--- include/apr_general.h.orig
+++ include/apr_general.h
@@ -76,7 +76,7 @@ typedef int               apr_signum_t;
  * @return offset
  */
 
-#if defined(CRAY) || (defined(__arm) && !(defined(LINUX) || defined(__FreeBSD__)))
+#if defined(CRAY) || (defined(__arm) && !(defined(LINUX) || defined(__FreeBSD__) || defined(__OpenBSD__)))
 #ifdef __STDC__
 #define APR_OFFSET(p_type,field) _Offsetof(p_type,field)
 #else