Try to fix clang build.

dlls/extdll.h

@@ -62,6 +62,7 @@ typedef int BOOL;
 // Misc C-runtime library headers
 #include "stdio.h"
 #include "stdlib.h"
+#include "stddef.h"
 #include "math.h"
 
 #if defined(__LP64__) || defined(__LLP64__) || defined(_WIN64) || (defined(__x86_64__) && !defined(__ILP32__) ) || defined(_M_X64) || defined(__ia64) || defined (_M_IA64) || defined(__aarch64__) || defined(__powerpc64__)

engine/eiface.h

@@ -355,7 +355,7 @@ typedef enum _fieldtypes
 	FIELD_TYPECOUNT		// MUST BE LAST
 } FIELDTYPE;
 
-#if !defined(offsetof) && !defined(GNUC)
+#if !defined(offsetof) && !defined(__GNUC__)
 #define offsetof(s,m)	(size_t)&(((s *)0)->m)
 #endif