Makefile.am: DISTCHECK_CONFIGURE_FLAGS -> AM_DISTCHECK_CONFIGURE_FLAGS

Makefile.am: DISTCHECK_CONFIGURE_FLAGS -> AM_DISTCHECK_CONFIGURE_FLAGS

The Automake manual states: "The user can still extend or override
the flags provided there by defining the DISTCHECK_CONFIGURE_FLAGS
variable". Overriding variable DISTCHECK_CONFIGURE_FLAGS in Makefile.am
makes it impossible for the user to do that. I discovered this
when trying to pass --enable-internal-jimtcl to distcheck.

Change-Id: Ibe5b1f23ccf3fbaa21c48b574a1b3f3e9f6fb916
Signed-off-by: R. Diez <rdiezmail-openocd@yahoo.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/8724
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins