configure: Define _WIN64 when building on a 64-bit platform.
diff --git a/include/basetsd.h b/include/basetsd.h
index 292483a..444e876 100644
--- a/include/basetsd.h
+++ b/include/basetsd.h
@@ -38,7 +38,7 @@
  * type model where int and long are 32 bit and pointer is 64-bit.
  */
 
-#ifdef __x86_64__
+#if defined(__x86_64__) && !defined(_WIN64)
 #define _WIN64
 #endif
 
diff --git a/include/windef.h b/include/windef.h
index 2908cfc..d9c0331 100644
--- a/include/windef.h
+++ b/include/windef.h
@@ -45,7 +45,7 @@
 # define __i386__
 #endif
 
-#ifdef __x86_64__
+#if defined(__x86_64__) && !defined(_WIN64)
 #define _WIN64
 #endif