configure: hide build issue of amt_jtagaccel driver by disabling it

configure: hide build issue of amt_jtagaccel driver by disabling it

With commit d8a2f6dbcf5f ("configure.ac: show the Amontec
JTAG-Accelerator driver in the config summary") the driver
amt_jtagaccel is now build by default on Linux.
This highlights the dependency of some include files, dependency
that is not properly managed and that can cause build failure.

The driver is queued to be dropped soon, so there is no real
interest to fix the dependencies.

Change the default so the driver is not built if the user does not
require it at configure time.

Change-Id: Ifb74e2c802abda290efbf59ca4ce02048c94e6f8
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8939
Reviewed-by: R. Diez <rdiez-2006@rd10.de>
Tested-by: jenkins