Add -FS flag to support parallel builds with MSVC 2013
diff --git a/configure.ac b/configure.ac
index 5aeaa08..045f62e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -141,6 +141,7 @@
     JE_CFLAGS_APPEND([-Zi])
     JE_CFLAGS_APPEND([-MT])
     JE_CFLAGS_APPEND([-W3])
+    JE_CFLAGS_APPEND([-FS])
     CPPFLAGS="$CPPFLAGS -I${srcroot}/include/msvc_compat"
   fi
 fi