Config.h: Add another definition which the Darwin build (sometimes) uses.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154630 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Config/config.h.in

@@ -15,4 +15,7 @@
 /* Directories clang will search for headers */
 #undef C_INCLUDE_DIRS
 
+/* Linker version detected at compile time. */
+#undef HOST_LINK_VERSION
+
 #endif