fix testsuite/48506

[Mike Stump]

I've checked in this to avoid problems with ports that default to -fno-common.  
I've also checked it into 4.6.x.


        PR testsuite/48506
        * gcc.dg/tree-ssa/ssa-ccp-17.c: Robustify against ports that
        default to fno-common.

Index: gcc.dg/tree-ssa/ssa-ccp-17.c
===================================================================
--- gcc.dg/tree-ssa/ssa-ccp-17.c        (revision 172125)
+++ gcc.dg/tree-ssa/ssa-ccp-17.c        (working copy)
@@ -18,7 +18,7 @@
   return ((struct Foo *)p)->i;
 }
 
-const struct Foo g;
+extern const struct Foo g;
 
 int foobar(void)
 {