MDEV-10725 Server 10.1.17 fails to build using clang with c++11

my_offsetof() returns a difference of two pointers, it must use the
appropriate return type (my_ptrdiff_t, not size_t)

include/my_global.h

@@ -857,8 +857,7 @@ typedef long long	my_ptrdiff_t;
   and related routines are refactored.
 */
 
-#define my_offsetof(TYPE, MEMBER) \
-        ((size_t)((char *)&(((TYPE *)0x10)->MEMBER) - (char*)0x10))
+#define my_offsetof(TYPE, MEMBER) PTR_BYTE_DIFF(&((TYPE *)0x10)->MEMBER, 0x10)
 
 #define NullS		(char *) 0
 

sql/parse_file.h

@@ -42,9 +42,9 @@ enum file_opt_type {
 
 struct File_option
 {
-  LEX_STRING name;		/**< Name of the option */
-  int offset;			/**< offset to base address of value */
-  file_opt_type type;		/**< Option type */
+  LEX_STRING name;              /**< Name of the option */
+  my_ptrdiff_t offset;          /**< offset to base address of value */
+  file_opt_type type;           /**< Option type */
 };