diff -up ClanLib-1.0.0/Sources/API/Core/System/cl_assert.h~ ClanLib-1.0.0/Sources/API/Core/System/cl_assert.h
--- ClanLib-1.0.0/Sources/API/Core/System/cl_assert.h~	2006-10-27 03:55:01.000000000 +0200
+++ ClanLib-1.0.0/Sources/API/Core/System/cl_assert.h	2018-05-29 14:54:12.132106898 +0200
@@ -33,6 +33,8 @@
 #ifndef header_cl_assert
 #define header_cl_assert
 
+#include <cstddef>
+
 #ifdef CL_API_DLL
 #ifdef CL_CORE_EXPORT
 #define CL_API_CORE __declspec(dllexport)