Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt	(revision 64408)
+++ CMakeLists.txt	(working copy)
@@ -55,6 +55,12 @@
 
 add_definitions(-D__REACTOS__)
 
+if(MSVC)
+    if(ARCH STREQUAL "amd64")
+        add_definitions(-D_LP64)
+    endif()
+endif()
+
 if(NOT CMAKE_CROSSCOMPILING)
 
     add_definitions(-DTARGET_${ARCH})