autotools: generate list of NEWS-xx file during configure

autotools: generate list of NEWS-xx file during configure

Commit 4fc0f3530c51 ("Makefile.am: fix non-POSIX warning from
automake") uses the BSD make extension '!=' to generate the list
of files NEWS-xx, but it's not POSIX (yet), it's not backward
compatible with GNU make 3.82 (used in CentOS 7, EOL scheduled for
2024) and depending on automake evolution it could trigger again
the warning 'apparently' just fixed.

Move in configure the generation of the file list.
The only drawback is that when a developer adds a new NEWS-xx
file, he/she has to run configure again before 'make distcheck',
otherwise will get failure.

Change-Id: Ia97e7f4e612655a97702f95e8451040539659b85
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Jan Matyáš <jmatyas@codasip.com>
Reviewed-on: http://openocd.zylin.com/6054
Tested-by: jenkins
Reviewed-by: Jan Matyas <matyas@codasip.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>