more robust build: prevent problems seen with Poly/ML eb94e2820013 on Mac OS X;

--- a/src/Pure/Admin/build_polyml.scala	Thu Mar 08 11:46:37 2018 +0100
+++ b/src/Pure/Admin/build_polyml.scala	Thu Mar 08 14:12:25 2018 +0100
@@ -116,7 +116,8 @@
     /* configure and make */
 
     val configure_options =
-      List("--enable-intinf-as-int", "--with-gmp") ::: info.platform_options(arch_64) ::: options
+      List("--disable-shared", "--enable-intinf-as-int", "--with-gmp") :::
+        info.platform_options(arch_64) ::: options
 
     bash(root,
       info.setup + "\n" +