Not sure if this is right, but OCAML_STDLIB_DIR can be undefined
in our cross-compiler.  It's only used in a one place in the C
code so this works around it.

diff --exclude _build -urN ocaml-3.11.0+beta1.orig/byterun/dynlink.c ocaml-3.11.0+beta1.mingw/byterun/dynlink.c
--- ocaml-3.11.0+beta1.orig/byterun/dynlink.c	2008-04-22 13:24:10.000000000 +0100
+++ ocaml-3.11.0+beta1.mingw/byterun/dynlink.c	2008-11-15 15:43:52.000000000 +0000
@@ -80,7 +80,12 @@
 
   stdlib = getenv("OCAMLLIB");
   if (stdlib == NULL) stdlib = getenv("CAMLLIB");
-  if (stdlib == NULL) stdlib = OCAML_STDLIB_DIR;
+  if (stdlib == NULL)
+#ifdef OCAML_STDLIB_DIR
+    stdlib = OCAML_STDLIB_DIR;
+#else
+    stdlib = ".";
+#endif
   ldconfname = caml_stat_alloc(strlen(stdlib) + 2 + sizeof(LD_CONF_NAME));
   strcpy(ldconfname, stdlib);
   strcat(ldconfname, "/" LD_CONF_NAME);