scripts/gdb: If not building expat for gdb, disable

--with-expat=yes is unconditionally passed to the gdb configure
stage, instead of respecting the ${do_expat} decision.

Disable if not needed. Prevents error building canadian cross;

configure: error: expat is missing or unusable

Where configure stage fails to find expat on the host compiler.

Signed-off-by: "David Holsgrove" <david.holsgrove@xilinx.com>
Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
Message-Id: <4c4410a2a8aab24a29c5.1349244128@localhost.localdomain>
PatchWork-Id: 188711

 diff -dur binutils-2.17.old/bfd/opncls.c binutils-2.17/bfd/opncls.c

 --- binutils-2.17.old/bfd/opncls.c	2006-03-16 13:20:16.000000000 +0100

 +++ binutils-2.17/bfd/opncls.c	2007-05-01 18:26:11.000000000 +0200

 @@ -158,6 +158,13 @@

  {

    bfd *nbfd;

    const bfd_target *target_vec;

 +  struct stat s;

 +

 +  if (stat (filename, &s) == 0)

 +    if (S_ISDIR(s.st_mode)) {

 +      bfd_set_error (bfd_error_file_not_recognized);

 +      return NULL;

 +    }

    nbfd = _bfd_new_bfd ();

    if (nbfd == NULL)