diff --git a/acinclude.m4 b/acinclude.m4
index 5a008f0..e22de35 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -113,7 +113,14 @@ fi
 dnl MinGW needs -march=i686 for atomics
 dnl ===================================
 if test $HAVE_MSWINDOWS = yes ; then
-    CFLAGS="$CFLAGS -march=i686"
+    case "$target" in
+    *x86_64*)
+        CFLAGS="$CFLAGS -m64"
+        ;;
+    *)
+        CFLAGS="$CFLAGS -march=i686"
+        ;;
+    esac
 fi
 
 dnl Byte order