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

source "config/configure.in"

source "config/backend.in"

source "config/global.in"

source "config/target.in"

source "config/toolchain.in"

source "config/kernel.in"

source "config/binutils.in"

source "config/cc.in"

source "config/libc.in"

source "config/debug.in"

source "config/companion_libs.in"

source "config/companion_tools.in"

source "config/test_suite.in"