Fix 64-bit portability problem.

Patch from Jonathan Wakely. Original explanation follows:

   The attached patch fixes a 64 bit portability problem where
   std::string::size_type is assigned to unsigned, which is shorter
   than size_t on x86-64 and so will be truncated. This means the
   following comparison to std::string::npos is always false.


[SVN r27772]

libs/program_options

@@ -1 +1 @@
-Subproject commit c2442fcad61dc48c0ce789f58b9df9d5feea2afc
+Subproject commit cb9bd037d946c4c18bad5a8e32fe2bf354c2e5c8