Index: src/Makefile
==================================================================
--- src/Makefile
+++ src/Makefile
@@ -172,8 +172,8 @@
 		 ${EXCEPTIONS_EXCEPTIONS_LIB_A}	\
 		 ${FORWARDING_FORWARDING_LIB_A}
 
 include ../buildsys.mk
 
-CPPFLAGS += -I. -I.. -Iexceptions -Iruntime
+CPPFLAGS += -I. -I.. -Iexceptions -Iruntime -DOF_COMPILING_OBJFW
 LD = ${OBJC}
 LDFLAGS += ${REEXPORT_LIBOBJC}

Index: src/OFObject.h
==================================================================
--- src/OFObject.h
+++ src/OFObject.h
@@ -28,10 +28,11 @@
 #include <stdbool.h>
 #include <limits.h>
 
 #import "macros.h"
 #import "autorelease.h"
+#import "block.h"
 
 OF_ASSUME_NONNULL_BEGIN
 
 /*! @file */
 

Index: src/block.h
==================================================================
--- src/block.h
+++ src/block.h
@@ -39,10 +39,22 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
 extern void* _Block_copy(const void*);
 extern void _Block_release(const void*);
+
+# if defined(OF_WINDOWS) && defined(OF_COMPILING_OBJFW)
+/*
+ * Clang has implicit declarations for these, but they are dllimport. When
+ * compiling ObjFW itself, these need to be dllexport.
+ */
+extern __declspec(dllexport) struct objc_abi_class _NSConcreteStackBlock;
+extern __declspec(dllexport) struct objc_abi_class _NSConcreteGlobalBlock;
+extern __declspec(dllexport) void _Block_object_assign(void*, const void*,
+    const int);
+extern __declspec(dllexport) void _Block_object_dispose(const void*, const int);
+# endif
 #ifdef __cplusplus
 }
 #endif
 
 #ifndef Block_copy

Index: src/exceptions/Makefile
==================================================================
--- src/exceptions/Makefile
+++ src/exceptions/Makefile
@@ -67,6 +67,6 @@
 
 INCLUDES = ${SRCS:.m=.h}
 
 include ../../buildsys.mk
 
-CPPFLAGS += -I. -I.. -I../.. -I../runtime
+CPPFLAGS += -I. -I.. -I../.. -I../runtime -DOF_COMPILING_OBJFW

Index: src/runtime/Makefile
==================================================================
--- src/runtime/Makefile
+++ src/runtime/Makefile
@@ -27,6 +27,6 @@
 OBJS_EXTRA = ${LOOKUP_ASM_LOOKUP_ASM_A}
 LIB_OBJS_EXTRA = ${LOOKUP_ASM_LOOKUP_ASM_LIB_A}
 
 include ../../buildsys.mk
 
-CPPFLAGS += -I. -I.. -I../..
+CPPFLAGS += -I. -I.. -I../.. -DOF_COMPILING_OBJFW